fix: normalize bare postgres:// to postgresql+psycopg:// (worker crash-loop)

Root cause (confirmed on local): even with psycopg installed, SQLAlchemy
raises:
  NoSuchModuleError: Can't load plugin: sqlalchemy.dialects:postgres
when DATABASE_URL uses the bare 'postgres://' scheme, because SQLAlchemy
only resolves 'postgresql+driver://'. The deploy's DATABASE_URL was
'postgres://...', so alembic upgrade head (run by the entrypoint before
starting services) crashed and the worker crash-looped in supervisor.

Fix:
- create_database_engine now normalizes 'postgres://' and legacy
  'postgres+pq://' to 'postgresql+psycopg://' so a bare postgres scheme
  works as long as psycopg is installed.
- Dockerfile build step now verifies psycopg imports after 'uv sync'
  (fails the build loudly instead of a runtime crash-loop).
- Tests: 4 for URL normalization; backend suite now 201 passed.

backend/app/db.py

@@ -22,6 +22,15 @@ def create_database_engine(database_url: str | None = None, **kwargs) -> Engine:
         data_dir.mkdir(parents=True, exist_ok=True)
         url = f"sqlite+pysqlite:///{(data_dir / 'crowdsight.db').resolve()}"
 
+    # Normalize bare `postgres://` (dialect alias SQLAlchemy only resolves in
+    # some versions) into the explicit psycopg3 dialect. Without this,
+    # create_engine raises "NoSuchModuleError: Can't load plugin:
+    # sqlalchemy.dialects:postgres" even though psycopg is installed.
+    if url.startswith("postgres://"):
+        url = url.replace("postgres://", "postgresql+psycopg://", 1)
+    elif url.startswith("postgres+pq://"):  # legacy psycopg2 scheme
+        url = url.replace("postgres+pq://", "postgresql+psycopg://", 1)
+
     connect_args = dict(kwargs.pop("connect_args", {}))
     if url.startswith("sqlite"):
         connect_args.setdefault("check_same_thread", False)