fix(config): normalize the postgres:// short scheme for the async ORM engine (#4293)

`app_sqlalchemy_url` rewrites `postgresql://` to `postgresql+asyncpg://` but
leaves libpq's `postgres://` short scheme untouched, so it reaches
`create_async_engine` verbatim and raises
`NoSuchModuleError: Can't load plugin: sqlalchemy.dialects:postgres`.

The two consumers of the same `database.postgres_url` disagree about that
scheme, which is what makes this a partial, backend-only failure rather than a
clean config error:

  - the checkpointer and store pass the raw URL to psycopg
    (`runtime/checkpointer/provider.py`, `runtime/store/provider.py`), and
    psycopg's `conninfo_to_dict` accepts `postgres://` and `postgresql://`
    identically;
  - the application ORM engine goes through `app_sqlalchemy_url`
    (`persistence/engine.py:181`), and SQLAlchemy dropped the `postgres`
    dialect alias in 2.0.

So a `postgres://` DSN brings the checkpointer up and takes the ORM engine
down. The `postgres_url` field docstring already promises the opposite --
"the +asyncpg driver suffix is added automatically where needed".

`postgres://` is a legal libpq URI scheme, not a typo, and is the form
`$DATABASE_URL` commonly takes on managed Postgres providers -- which is
exactly what the module docstring recommends configuring
(`postgres_url: $DATABASE_URL`).

Normalize it alongside `postgresql://`. The existing tests covered
`postgresql://` and `postgresql+asyncpg://` but not the short scheme; the new
case fails on main with the `NoSuchModuleError` above.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/packages/harness/deerflow/config/database_config.py

@@ -98,5 +98,8 @@ class DatabaseConfig(BaseModel):
             url = self.postgres_url
             if url.startswith("postgresql://"):
                 url = url.replace("postgresql://", "postgresql+asyncpg://", 1)
+            elif url.startswith("postgres://"):
+                # libpq's short alias: accepted by the psycopg checkpointer, but not a SQLAlchemy dialect.
+                url = url.replace("postgres://", "postgresql+asyncpg://", 1)
             return url
         raise ValueError(f"No SQLAlchemy URL for backend={self.backend!r}")

backend/tests/test_persistence_scaffold.py

@@ -49,6 +49,15 @@ class TestDatabaseConfig:
         assert url.startswith("postgresql+asyncpg://")
         assert "u:p@h:5432/db" in url
 
+    def test_app_sqlalchemy_url_postgres_short_scheme(self):
+        c = DatabaseConfig(
+            backend="postgres",
+            postgres_url="postgres://u:p@h:5432/db",
+        )
+        url = c.app_sqlalchemy_url
+        assert url.startswith("postgresql+asyncpg://")
+        assert "u:p@h:5432/db" in url
+
     def test_app_sqlalchemy_url_postgres_already_asyncpg(self):
         c = DatabaseConfig(
             backend="postgres",