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/frontend/flask_app/tests/conftest.py

@@ -0,0 +1,104 @@
+"""Shared test fixtures for Flask client tests.
+
+Creates a Flask test client and mocks the API client (requests to FastAPI server).
+"""
+import sys
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+# Ensure the client package is importable
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from app import create_app
+
+# All blueprint modules that import api_client at module level.
+# We must patch at the *use-site* (blueprint namespace) so the local name
+# ``api_client`` in each module points to the mock, not the real singleton.
+_API_CLIENT_PATCH_TARGETS = [
+    "blueprints.auth.api_client",
+    "blueprints.maker.api_client",
+    "blueprints.measure.api_client",
+    "blueprints.statistics.api_client",
+]
+
+
+@pytest.fixture
+def flask_app():
+    """Create a Flask application configured for testing."""
+    app = create_app()
+    app.config["TESTING"] = True
+    app.config["WTF_CSRF_ENABLED"] = False
+    app.config["SECRET_KEY"] = "test-secret-key"
+
+    # Provide default template variables that are normally set inside
+    # specific Jinja2 blocks and therefore not visible to sibling blocks.
+    @app.context_processor
+    def _inject_template_defaults():
+        return {
+            "versions": [],
+            "current_version": None,
+        }
+
+    return app
+
+
+@pytest.fixture
+def client(flask_app):
+    """Yield a Flask test client."""
+    with flask_app.test_client() as test_client:
+        yield test_client
+
+
+@pytest.fixture
+def logged_in_client(flask_app):
+    """Yield a Flask test client with a logged-in session."""
+    with flask_app.test_client() as test_client:
+        with test_client.session_transaction() as sess:
+            sess["api_key"] = "test-api-key-12345"
+            sess["user"] = {
+                "id": 1,
+                "username": "testuser",
+                "display_name": "Test User",
+                "roles": ["Maker", "MeasurementTec", "Metrologist"],
+                "is_admin": True,
+                "language_pref": "en",
+                "theme_pref": "light",
+                "active": True,
+            }
+            sess["user_id"] = 1
+            sess["language"] = "en"
+            sess["theme"] = "light"
+        yield test_client
+
+
+@pytest.fixture
+def mock_api_client():
+    """Patch the api_client singleton used in blueprints.
+
+    Patches every blueprint module that does
+    ``from services.api_client import api_client`` so the local name
+    in each module resolves to the same MagicMock instance.
+
+    Yields the mocked APIClient instance so tests can configure responses:
+
+        mock_api_client.get.return_value = {"items": [...], "total": 1}
+        mock_api_client.post.return_value = {"user": {...}, "api_key": "..."}
+    """
+    mock = MagicMock()
+    # Default: all methods return empty success dict
+    mock.get.return_value = {}
+    mock.post.return_value = {}
+    mock.put.return_value = {}
+    mock.delete.return_value = {}
+
+    # Stack patches for every blueprint that imports api_client
+    patchers = [patch(target, mock) for target in _API_CLIENT_PATCH_TARGETS]
+    for p in patchers:
+        p.start()
+
+    yield mock
+
+    for p in patchers:
+        p.stop()