chore(v2): restructure monorepo to src/ layout with uv

Aligns the repo with the python-project-spec-design.md template chosen
for V2.0.0. Big move, no logic changes. The 3 pre-existing test
failures (test_recipes::test_update_recipe, test_recipes::
test_recipe_versioning, test_tasks::test_reorder_tasks, plus the
client test_save_measurement_proxy) survive unchanged.

Layout changes
- server/        -> src/backend/
- server/middleware/ -> src/backend/api/middleware/
- server/routers/    -> src/backend/api/routers/
- server/models/     -> src/backend/models/orm/
- server/schemas/    -> src/backend/models/api/
- server/uploads/    -> uploads/ (project root, mounted volume)
- server/tests/      -> src/backend/tests/
- client/            -> src/frontend/flask_app/ (Flask kept; React
  deroga is documented in CLAUDE.md, justified by tablet UX, USB
  caliper/barcode workflow and Fabric.js integration)

Tooling
- pyproject.toml: monorepo with [project] core deps and
  optional-dependencies server / client / dev. Replaces both
  server/requirements.txt and client/requirements.txt.
- uv.lock + .python-version (3.11) committed for reproducible builds.
- Dockerfile (root, backend) and Dockerfile.frontend rewritten to use
  uv sync --frozen --no-dev --extra server|client; legacy Dockerfiles
  preserved as Dockerfile.legacy for reference but excluded from build
  context via .dockerignore.
- docker-compose.dev.yml + docker-compose.yml: build context now ".",
  dockerfile pointing to the root files.

Code adjustments forced by the move
- Every "from config|database|models|schemas|services|routers|middleware
  import ..." rewritten to its src.backend.* equivalent (50+ files
  including indented inline imports inside test bodies).
- src/backend/migrations/env.py: insert project root into sys.path so
  alembic can resolve src.backend.* imports regardless of cwd.
- src/backend/config.py: env_file ../../.env (was ../.env), upload_path
  resolves project root via parents[2].
- src/backend/tests/conftest.py + tests: import ... from src.backend.*
  instead of bare names; old per-directory pytest.ini files removed in
  favor of root pyproject.toml [tool.pytest.ini_options].
- .gitignore: uploads/ at root, src/frontend/flask_app/static/css/
  tailwind.css path; .dockerignore tightened.
- CLAUDE.md: rewrote sections "Layout del repository", "Comandi di
  Sviluppo", "Database & Migrations", "Test", "i18n", and all path
  references throughout the architecture sections.

Verified
- uv lock resolves 77 packages; uv sync --extra server --extra client
  --extra dev installs cleanly.
- uv run pytest: 171 passed, 4 pre-existing failures.
- uv run alembic -c src/backend/migrations/alembic.ini check loads
  config and metadata (errors only on the absent local MySQL).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/backend/api/routers/auth.py

@@ -0,0 +1,62 @@
+"""Authentication router - login, logout, profile."""
+from fastapi import APIRouter, Depends, HTTPException, status
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from src.backend.database import get_db
+from src.backend.api.middleware.api_key import get_current_user
+from src.backend.models.orm.user import User
+from src.backend.models.api.user import (
+    LoginRequest,
+    LoginResponse,
+    UserProfileUpdate,
+    UserResponse,
+)
+from src.backend.services.auth_service import authenticate_user, login_user, logout_user
+
+router = APIRouter(prefix="/api/auth", tags=["auth"])
+
+
+@router.post("/login", response_model=LoginResponse)
+async def login(data: LoginRequest, db: AsyncSession = Depends(get_db)):
+    """Login with username and password, receive API key."""
+    user = await authenticate_user(db, data.username, data.password)
+    if user is None:
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="Invalid username or password",
+        )
+    api_key = await login_user(db, user)
+    return LoginResponse(
+        user=UserResponse.model_validate(user),
+        api_key=api_key,
+    )
+
+
+@router.post("/logout", status_code=status.HTTP_204_NO_CONTENT)
+async def logout(
+    user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    """Logout - invalidate API key."""
+    await logout_user(db, user)
+
+
+@router.get("/me", response_model=UserResponse)
+async def get_profile(user: User = Depends(get_current_user)):
+    """Get current user profile."""
+    return UserResponse.model_validate(user)
+
+
+@router.put("/me", response_model=UserResponse)
+async def update_profile(
+    data: UserProfileUpdate,
+    user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    """Update own profile (display_name, language, theme)."""
+    update_data = data.model_dump(exclude_unset=True)
+    for field, value in update_data.items():
+        setattr(user, field, value)
+    await db.flush()
+    await db.refresh(user)
+    return UserResponse.model_validate(user)