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/services/auth_service.py

@@ -0,0 +1,97 @@
+"""Authentication service - password hashing, API key management."""
+import secrets
+from datetime import datetime
+
+import bcrypt
+from sqlalchemy import select, update
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from src.backend.models.orm.user import User
+
+
+def hash_password(password: str) -> str:
+    """Hash a password using bcrypt."""
+    return bcrypt.hashpw(password.encode("utf-8"), bcrypt.gensalt()).decode("utf-8")
+
+
+def verify_password(plain_password: str, hashed_password: str) -> bool:
+    """Verify a password against its hash."""
+    return bcrypt.checkpw(
+        plain_password.encode("utf-8"), hashed_password.encode("utf-8")
+    )
+
+
+def generate_api_key() -> str:
+    """Generate a secure 64-character API key."""
+    return secrets.token_urlsafe(48)  # 64 chars base64
+
+
+async def authenticate_user(
+    db: AsyncSession, username: str, password: str
+) -> User | None:
+    """Authenticate user by username and password."""
+    result = await db.execute(
+        select(User).where(User.username == username, User.active == True)
+    )
+    user = result.scalar_one_or_none()
+    if user is None or not verify_password(password, user.password_hash):
+        return None
+    return user
+
+
+async def login_user(db: AsyncSession, user: User) -> str:
+    """Generate API key and update last_login for user."""
+    api_key = generate_api_key()
+    await db.execute(
+        update(User)
+        .where(User.id == user.id)
+        .values(api_key=api_key, last_login=datetime.utcnow())
+    )
+    await db.flush()
+    return api_key
+
+
+async def logout_user(db: AsyncSession, user: User) -> None:
+    """Invalidate user's API key."""
+    await db.execute(
+        update(User).where(User.id == user.id).values(api_key=None)
+    )
+    await db.flush()
+
+
+async def create_user(
+    db: AsyncSession,
+    username: str,
+    password: str,
+    display_name: str,
+    email: str | None = None,
+    roles: list[str] | None = None,
+    is_admin: bool = False,
+    language_pref: str = "it",
+    theme_pref: str = "light",
+) -> User:
+    """Create a new user with hashed password."""
+    user = User(
+        username=username,
+        password_hash=hash_password(password),
+        display_name=display_name,
+        email=email,
+        roles=roles or [],
+        is_admin=is_admin,
+        language_pref=language_pref,
+        theme_pref=theme_pref,
+    )
+    db.add(user)
+    await db.flush()
+    await db.refresh(user)
+    return user
+
+
+async def regenerate_api_key(db: AsyncSession, user_id: int) -> str:
+    """Regenerate API key for a user."""
+    new_key = generate_api_key()
+    await db.execute(
+        update(User).where(User.id == user_id).values(api_key=new_key)
+    )
+    await db.flush()
+    return new_key