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/templates/errors/station_not_configured.html

@@ -0,0 +1,33 @@
+{% extends "base.html" %}
+{% block title %}{{ _('Stazione non configurata') }} — TieMeasureFlow{% endblock %}
+
+{% block content %}
+<div class="container mx-auto px-4 py-8 max-w-2xl">
+  <div class="mt-20 p-8 bg-[var(--bg-card)] rounded-xl shadow-lg text-center border border-red-200 dark:border-red-800">
+    <div class="inline-flex items-center justify-center w-16 h-16 rounded-full
+                bg-red-50 dark:bg-red-900/30 mb-6">
+      <svg class="w-8 h-8 text-measure-fail" fill="none" stroke="currentColor" stroke-width="1.75" viewBox="0 0 24 24">
+        <path stroke-linecap="round" stroke-linejoin="round"
+              d="M12 9v3.75m-9.303 3.376c-.866 1.5.217 3.374 1.948 3.374h14.71c1.73 0 2.813-1.874 1.948-3.374L13.949 3.378c-.866-1.5-3.032-1.5-3.898 0L2.697 16.126zM12 15.75h.007v.008H12v-.008z"/>
+      </svg>
+    </div>
+
+    <h1 class="text-2xl font-bold text-measure-fail mb-4">
+      {{ _('Stazione non configurata') }}
+    </h1>
+
+    <p class="mb-4 text-[var(--text-primary)]">
+      {{ _('Questo client non ha impostato la variabile di ambiente STATION_CODE.') }}
+    </p>
+
+    <p class="mb-6 text-sm text-[var(--text-secondary)]">
+      {{ _('Contattare il responsabile IT: il file .env del container deve contenere STATION_CODE con il codice della stazione assegnata.') }}
+    </p>
+
+    {% if error %}
+    <pre class="text-xs text-left text-[var(--text-secondary)] bg-[var(--bg-secondary)]
+                p-3 rounded-lg border border-[var(--border-color)] overflow-auto">{{ error }}</pre>
+    {% endif %}
+  </div>
+</div>
+{% endblock %}