Backend tests run on SQLite by default (78 passed, 1 skipped); no Postgres needed

- conftest: default SHONAR_TEST_DATABASE_URL is a temp SQLite file, matching
  the bundled-lite engine; set the env var to a PG URL to exercise that path
- models.py: add sqlite_where to the two partial unique indexes — without it
  SQLite built a FULL unique index on recording_id (WHERE not carried over),
  wrongly blocking a second export asset per recording
- test_m9: pass UUID objects (not str) to direct ORM inserts/gets; SQLite's
  GUID bind processor rejects strings (asyncpg tolerated them)

Verified: pytest -q = 78 passed, 1 skipped; ruff check clean

backend/tests/conftest.py

@@ -1,7 +1,10 @@
 """Pytest fixtures.
 
-Tests run against a real PostgreSQL (deploy/docker-compose.dev.yml) using a
-dedicated ``shonar_test`` database, plus a temporary local storage root.
+Default: SQLite (aiosqlite) in a temp dir — matches the desktop
+bundled-lite engine and needs no services running. Set
+``SHONAR_TEST_DATABASE_URL`` to a PostgreSQL URL (e.g.
+``postgresql+asyncpg://shonar:shonar@localhost:5432/shonar_test``) to
+exercise the Postgres path, including the tsvector FTS columns.
 """
 
 from __future__ import annotations
@@ -15,9 +18,11 @@ import pytest
 import pytest_asyncio
 
 # Configure env BEFORE importing the app so Settings picks it up.
+_db_dir = tempfile.mkdtemp(prefix="shonar-test-db-")
 TEST_DB = os.environ.get(
     "SHONAR_TEST_DATABASE_URL",
-    "postgresql+asyncpg://shonar:shonar@localhost:5432/shonar_test",
+    # Bundled-lite default: throwaway SQLite file; no services required.
+    f"sqlite+aiosqlite:///{os.path.join(_db_dir, 'test.db')}",
 )
 os.environ["SHONAR_DATABASE_URL"] = TEST_DB
 os.environ["SHONAR_SECRET_KEY"] = "test-secret-key-0123456789abcdef0123456789abcdef"