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/components/next_measurement.html

@@ -0,0 +1,32 @@
+{# Next Measurement Component
+
+   Indicatore della prossima misura da effettuare.
+
+   Variabili Alpine.js richieste nel parent component:
+   - nextSubtask: object | null - prossimo subtask da misurare
+     - marker_number: int
+     - description: string
+     - nominal: float
+     - unit: string
+#}
+
+<div x-show="nextSubtask" class="mt-4 p-3 rounded-lg bg-slate-50 dark:bg-slate-800 border border-slate-200 dark:border-slate-700">
+  <div class="flex items-center gap-2 text-sm text-steel dark:text-steel-light">
+    <!-- Arrow right icon -->
+    <svg class="w-4 h-4 text-primary flex-shrink-0" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+      <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M13 7l5 5m0 0l-5 5m5-5H6"/>
+    </svg>
+    <span class="font-medium">{{ _('Prossima misura') }}:</span>
+    <span class="font-semibold text-[var(--text-primary)]">
+      #<span x-text="nextSubtask?.marker_number"></span>
+      <span x-text="nextSubtask?.description" class="ml-1"></span>
+    </span>
+  </div>
+
+  <!-- Nominal preview -->
+  <div class="mt-1 ml-6 text-xs text-steel dark:text-steel-light">
+    <span>Nom: </span>
+    <span class="font-mono font-medium" x-text="nextSubtask?.nominal?.toFixed(3)"></span>
+    <span class="ml-1" x-text="nextSubtask?.unit || 'mm'"></span>
+  </div>
+</div>