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/models/orm/station.py

@@ -0,0 +1,70 @@
+"""Station and StationRecipeAssignment models.
+
+A Station represents a physical control point (typically one per tablet/PC).
+Recipes are assigned to stations so that each station only sees the products
+it is supposed to inspect.
+"""
+from datetime import datetime
+from typing import TYPE_CHECKING, Optional
+
+from sqlalchemy import Boolean, DateTime, ForeignKey, Integer, String, Text, UniqueConstraint, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from src.backend.database import Base
+
+if TYPE_CHECKING:
+    from src.backend.models.orm.recipe import Recipe
+
+
+class Station(Base):
+    __tablename__ = "stations"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    code: Mapped[str] = mapped_column(String(100), nullable=False, index=True)
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    location: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)
+    notes: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
+    active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True, index=True)
+    created_by: Mapped[int] = mapped_column(Integer, ForeignKey("users.id"), nullable=False)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime, nullable=False, server_default=func.now()
+    )
+
+    assignments: Mapped[list["StationRecipeAssignment"]] = relationship(
+        back_populates="station", cascade="all, delete-orphan", lazy="selectin"
+    )
+
+    __table_args__ = (
+        UniqueConstraint("code", name="uq_stations_code"),
+        {"mysql_engine": "InnoDB", "mysql_charset": "utf8mb4"},
+    )
+
+    def __repr__(self) -> str:
+        return f"<Station {self.code} '{self.name}'>"
+
+
+class StationRecipeAssignment(Base):
+    __tablename__ = "station_recipe_assignments"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    station_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("stations.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    recipe_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("recipes.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    assigned_by: Mapped[int] = mapped_column(Integer, ForeignKey("users.id"), nullable=False)
+    assigned_at: Mapped[datetime] = mapped_column(
+        DateTime, nullable=False, server_default=func.now()
+    )
+
+    station: Mapped["Station"] = relationship(back_populates="assignments")
+    recipe: Mapped["Recipe"] = relationship(lazy="selectin")
+
+    __table_args__ = (
+        UniqueConstraint("station_id", "recipe_id", name="uq_station_recipe"),
+        {"mysql_engine": "InnoDB", "mysql_charset": "utf8mb4"},
+    )
+
+    def __repr__(self) -> str:
+        return f"<StationRecipeAssignment station={self.station_id} recipe={self.recipe_id}>"