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/middleware/security_headers.py

@@ -0,0 +1,42 @@
+"""Security headers middleware for FastAPI.
+
+Adds standard security headers to every HTTP response to mitigate
+common web vulnerabilities (clickjacking, XSS, MIME sniffing, etc.).
+"""
+from typing import Callable
+
+from fastapi import Request, Response
+from starlette.middleware.base import BaseHTTPMiddleware
+
+from src.backend.config import settings
+
+# Content Security Policy - allows CDN resources used by the client
+# Note: 'unsafe-eval' required for Plotly.js runtime evaluation in SPC charts
+CSP = (
+    "default-src 'self'; "
+    "script-src 'self' 'unsafe-inline' 'unsafe-eval' "
+    "https://cdn.tailwindcss.com https://cdn.jsdelivr.net https://cdn.plot.ly; "
+    "style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; "
+    "font-src 'self' https://fonts.gstatic.com; "
+    "img-src 'self' data: blob:; "
+    "connect-src 'self'"
+)
+
+
+class SecurityHeadersMiddleware(BaseHTTPMiddleware):
+    """Middleware that injects security headers into every response."""
+
+    async def dispatch(self, request: Request, call_next: Callable) -> Response:
+        response = await call_next(request)
+
+        response.headers["X-Content-Type-Options"] = "nosniff"
+        response.headers["X-Frame-Options"] = "DENY"
+        response.headers["X-XSS-Protection"] = "1; mode=block"
+        response.headers["Referrer-Policy"] = "strict-origin-when-cross-origin"
+        response.headers["Content-Security-Policy"] = CSP
+
+        # Add HSTS header only when running with HTTPS (SSL configured)
+        if settings.ssl_certfile and settings.ssl_keyfile:
+            response.headers["Strict-Transport-Security"] = "max-age=31536000; includeSubDomains"
+
+        return response