M2-T11: serve React SPA from FastAPI and remove Jinja pages

- app/main.py serves the SPA build (SPA_DIST_DIR, default frontend/dist):
  mounts /assets and a GET catch-all returning index.html for client routes;
  catch-all 404s on /api/*, never swallows /docs, /openapi.json, /static, assets,
  ingestion/ticktick/status; skips SPA serving when dist absent (backend-only CI)
- delete app/api/routes/pages.py, app/api/routes/auth.py, app/templates/
  (all replaced by /api/* + SPA; auth service layer kept)
- remove/replace Jinja page tests (JSON coverage already in test_api_*);
  add tests/test_spa_hosting.py for the fallback contract
- regenerate openapi/ (Jinja paths gone) and frontend schema.d.ts

tests/test_app.py

@@ -25,9 +25,11 @@ def _prepare_app_db(tmp_path) -> str:
 
 
 def test_app_starts(client: TestClient) -> None:
+    # With SPA enabled, GET / is served by the catch-all and returns index.html (200).
+    # Without SPA (e.g. SPA_DIST_DIR points to empty dir), it returns 404.
+    # Either way the app started successfully — just assert it is not a server error.
     response = client.get("/", follow_redirects=False)
-    assert response.status_code == 303
-    assert response.headers["location"] == "/login"
+    assert response.status_code in (200, 404)
 
 
 def test_status_endpoint(client: TestClient) -> None: