fix(tasks): copy-on-write su modifica/cancellazione e ordine tolleranze (#4, #26)

#4 - api/routers/tasks.py: update_task, delete_task, create_subtask,
update_subtask, delete_subtask controllavano solo is_current, non
version_has_measurements: la versione con misure veniva modificata sul posto,
cambiando i limiti di quote gia' giudicate pass/fail. Nuovi helper
_task_for_write / _subtask_for_write riusano recipe_service.create_new_version
(lo stesso copy-on-write di create_task): se la versione ha misure creano una
nuova versione, ritrovano il task per order_index e la quota per marker_number
(chiavi che _copy_tasks_to_version preserva), e la scrittura va sulla copia.
La versione misurata resta intatta.
Trovato e corretto nello stesso punto: il task ritrovato nella copia non ha
.version precaricato (new_version.tasks viene da selectinload, che non popola
il backref figlio->genitore), e TaskResponse.recipe_id legge task.version.recipe_id
- senza assegnare esplicitamente candidate.version = new_version, update_task
va in MissingGreenlet (lazy load fuori da un await) invece di rispondere 200.

#26 - models/api/task.py: nuovo validate_tolerance_order (ltl<=lwl<=nominal<=
uwl<=utl sui soli campi valorizzati), usato come model_validator su
SubtaskCreate e richiamato dal router in update_subtask sui valori risultanti
(esistenti + parziali) prima del flush, perche' SubtaskUpdate non ha un insieme
fisso di campi da validare da solo.

Test: src/backend/tests/test_tasks.py, TestCopyOnWriteOnEdit (5) e
TestToleranceOrder (4); adattato test_update_subtask esistente (nominal 12.0
violava il nuovo ordine contro utl=10.5 invariato).
PROVA: uv run --extra server --extra dev pytest src/backend/tests/test_tasks.py -q -> 16 passed

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/backend/api/routers/tasks.py

@@ -17,6 +17,7 @@ from src.backend.models.api.task import (
     TaskReorderRequest,
     TaskResponse,
     TaskUpdate,
+    validate_tolerance_order,
 )
 from src.backend.services import recipe_service
 
@@ -99,6 +100,64 @@ async def _ensure_version_is_current(db: AsyncSession, version_id: int) -> Recip
     return version
 
 
+async def _task_for_write(db: AsyncSession, task: RecipeTask, user: User) -> RecipeTask:
+    """Return the task to write to: the same one, or its copy in a new version.
+
+    A version that already has measurements cannot be edited on the spot - its
+    recorded pass/fail would silently start meaning something else. Copy-on-write
+    (the same one create_task/update_recipe use) gives the edit a fresh version
+    and leaves the measured one exactly as it was when the readings were taken.
+    The task is matched in the copy by its position (order_index), which
+    _copy_tasks_to_version preserves exactly.
+    """
+    if not await recipe_service.version_has_measurements(db, task.version_id):
+        return task
+
+    recipe_id = task.recipe_id
+    if recipe_id is None:
+        raise HTTPException(
+            status_code=status.HTTP_409_CONFLICT,
+            detail="Cannot determine the recipe for this task",
+        )
+    from src.backend.models.api.recipe import RecipeUpdate
+    new_version = await recipe_service.create_new_version(
+        db, recipe_id, RecipeUpdate(change_notes=f"Edited task: {task.title}"), user,
+    )
+    for candidate in new_version.tasks:
+        if candidate.order_index == task.order_index:
+            # TaskResponse.recipe_id reads task.version.recipe_id; new_version's
+            # own tasks are loaded via selectinload, which does not also
+            # populate the child->parent backref, so left alone this would
+            # lazy-load .version outside of an awaited context and crash.
+            candidate.version = new_version
+            return candidate
+    raise HTTPException(
+        status_code=status.HTTP_409_CONFLICT,
+        detail="Could not locate the task in the new version",
+    )
+
+
+async def _subtask_for_write(
+    db: AsyncSession, task: RecipeTask, subtask: RecipeSubtask, user: User,
+) -> tuple[RecipeTask, RecipeSubtask]:
+    """Return (task, subtask) to write to: the same ones, or their copies.
+
+    Same copy-on-write as _task_for_write, one level down: once the task's copy
+    is found, the quote is matched inside it by marker_number (unique per task).
+    """
+    new_task = await _task_for_write(db, task, user)
+    if new_task is task:
+        return task, subtask
+
+    for candidate in new_task.subtasks:
+        if candidate.marker_number == subtask.marker_number:
+            return new_task, candidate
+    raise HTTPException(
+        status_code=status.HTTP_409_CONFLICT,
+        detail="Could not locate the quote in the new version",
+    )
+
+
 # ---------------------------------------------------------------------------
 # Task endpoints
 # ---------------------------------------------------------------------------
@@ -264,10 +323,16 @@ async def reorder_tasks(
 async def update_task(
     task_id: int,
     data: TaskUpdate,
-    _user: User = Depends(require_maker),
+    user: User = Depends(require_maker),
     db: AsyncSession = Depends(get_db),
 ):
-    """Update a task. The task must belong to the current version. Requires Maker role."""
+    """Update a task. The task must belong to the current version. Requires Maker role.
+
+    If that version already has measurements the edit goes through copy-on-write:
+    a new version is created and its copy of the task is the one changed. The
+    version with measurements is left untouched, so its recorded pass/fail keeps
+    meaning what it meant when it was taken.
+    """
     task = await _get_task_or_404(db, task_id)
     await _ensure_version_is_current(db, task.version_id)
 
@@ -275,6 +340,8 @@ async def update_task(
     if not update_data:
         return TaskResponse.model_validate(task)
 
+    task = await _task_for_write(db, task, user)
+
     for field, value in update_data.items():
         setattr(task, field, value)
 
@@ -286,16 +353,19 @@ async def update_task(
 @router.delete("/api/tasks/{task_id}", status_code=204)
 async def delete_task(
     task_id: int,
-    _user: User = Depends(require_maker),
+    user: User = Depends(require_maker),
     db: AsyncSession = Depends(get_db),
 ):
     """Delete a task and its subtasks. The task must belong to the current version.
 
-    Requires Maker role.
+    Requires Maker role. Copy-on-write applies as in update_task: a version with
+    measurements is never touched, the deletion lands on its copy instead.
     """
     task = await _get_task_or_404(db, task_id)
     await _ensure_version_is_current(db, task.version_id)
 
+    task = await _task_for_write(db, task, user)
+
     await db.delete(task)
     await db.flush()
 
@@ -312,16 +382,19 @@ async def delete_task(
 async def create_subtask(
     task_id: int,
     data: SubtaskCreate,
-    _user: User = Depends(require_maker),
+    user: User = Depends(require_maker),
     db: AsyncSession = Depends(get_db),
 ):
     """Add a subtask to a task. The task must belong to the current version.
 
-    Requires Maker role.
+    Requires Maker role. Copy-on-write applies as in create_task: a version with
+    measurements gets a new version, and the subtask is added there instead.
     """
     task = await _get_task_or_404(db, task_id)
     await _ensure_version_is_current(db, task.version_id)
 
+    task = await _task_for_write(db, task, user)
+
     # Check marker_number uniqueness within the task
     existing_markers = {s.marker_number for s in task.subtasks}
     if data.marker_number in existing_markers:
@@ -331,7 +404,7 @@ async def create_subtask(
         )
 
     subtask = RecipeSubtask(
-        task_id=task_id,
+        task_id=task.id,
         marker_number=data.marker_number,
         description=data.description,
         measurement_type=data.measurement_type,
@@ -354,12 +427,13 @@ async def create_subtask(
 async def update_subtask(
     subtask_id: int,
     data: SubtaskUpdate,
-    _user: User = Depends(require_maker),
+    user: User = Depends(require_maker),
     db: AsyncSession = Depends(get_db),
 ):
     """Update a subtask. Its parent task must belong to the current version.
 
-    Requires Maker role.
+    Requires Maker role. Copy-on-write applies as in update_task, one level
+    down: a version with measurements is never touched, the quote's copy is.
     """
     subtask = await _get_subtask_or_404(db, subtask_id)
 
@@ -371,6 +445,24 @@ async def update_subtask(
     if not update_data:
         return SubtaskResponse.model_validate(subtask)
 
+    # SubtaskUpdate is partial, so the order can only be checked against the
+    # values the write would actually leave in place: existing + incoming.
+    def _resulting(field: str) -> float | None:
+        value = update_data.get(field, getattr(subtask, field))
+        return None if value is None else float(value)
+
+    try:
+        validate_tolerance_order(
+            _resulting("ltl"), _resulting("lwl"), _resulting("nominal"),
+            _resulting("uwl"), _resulting("utl"),
+        )
+    except ValueError as exc:
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_CONTENT, detail=str(exc),
+        )
+
+    task, subtask = await _subtask_for_write(db, task, subtask, user)
+
     for field, value in update_data.items():
         setattr(subtask, field, value)
 
@@ -382,12 +474,13 @@ async def update_subtask(
 @router.delete("/api/subtasks/{subtask_id}", status_code=204)
 async def delete_subtask(
     subtask_id: int,
-    _user: User = Depends(require_maker),
+    user: User = Depends(require_maker),
     db: AsyncSession = Depends(get_db),
 ):
     """Delete a subtask. Its parent task must belong to the current version.
 
-    Requires Maker role.
+    Requires Maker role. Copy-on-write applies as in update_task, one level
+    down: a version with measurements is never touched, the quote's copy is.
     """
     subtask = await _get_subtask_or_404(db, subtask_id)
 
@@ -395,5 +488,7 @@ async def delete_subtask(
     task = await _get_task_or_404(db, subtask.task_id)
     await _ensure_version_is_current(db, task.version_id)
 
+    task, subtask = await _subtask_for_write(db, task, subtask, user)
+
     await db.delete(subtask)
     await db.flush()