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.

Dockerfile

@@ -44,7 +44,8 @@ COPY locales ./locales
 COPY package.json ./
 
 # Install backend deps against the final pyproject.toml/uv.lock
-RUN cd backend && uv sync --frozen --no-dev
+RUN cd backend && uv sync --frozen --no-dev \
+  && uv run --frozen python -c "import psycopg" || (echo "FATAL: psycopg not installed — package sync missing the Postgres driver" >&2 && exit 1)
 
 # Make the migration-runner entrypoint executable
 RUN chmod +x /app/backend/docker_entrypoint.sh

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)

backend/tests/test_db_url_normalization.py

@@ -0,0 +1,42 @@
+"""Tests for create_database_engine URL normalization.
+
+Bare `postgres://` and legacy `postgres+pq://` schemes must be normalized to
+the explicit `postgresql+psycopg://` dialect, otherwise SQLAlchemy raises
+"NoSuchModuleError: Can't load plugin: sqlalchemy.dialects:postgres" even when
+psycopg is installed.
+"""
+
+from app.db import create_database_engine
+
+
+def test_normalizes_bare_postgres_scheme():
+    engine = create_database_engine("postgres://user:pass@db.example.com:5432/mydb")
+    try:
+        assert str(engine.url).startswith("postgresql+psycopg://")
+        assert "user:***@db.example.com:5432/mydb" in str(engine.url)
+    finally:
+        engine.dispose()
+
+
+def test_normalizes_legacy_psycopg2_scheme():
+    engine = create_database_engine("postgres+pq://user:pass@localhost:5432/db")
+    try:
+        assert str(engine.url).startswith("postgresql+psycopg://")
+    finally:
+        engine.dispose()
+
+
+def test_leaves_explicit_psycopg_scheme_unchanged():
+    engine = create_database_engine("postgresql+psycopg://user:pass@localhost:5432/db")
+    try:
+        assert str(engine.url).startswith("postgresql+psycopg://")
+    finally:
+        engine.dispose()
+
+
+def test_leaves_sqlite_unchanged():
+    engine = create_database_engine("sqlite:////tmp/db-test.sqlite")
+    try:
+        assert str(engine.url).startswith("sqlite:")
+    finally:
+        engine.dispose()