feat: comprehensive health check with DB, scheduler, cache

- Add /api/v1/health endpoint with component-level checks
- Verify DB connectivity, fail2ban socket, scheduler, session cache
- Add SQLite WAL cleanup on startup (orphan crash files)
- Migration 8: import_log.timestamp → INTEGER UNIX epoch
- Align import_log timestamps with history_archive (already UNIX int)
- Add unit tests for DB cleanup and health router

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/tests/conftest.py

@@ -62,12 +62,19 @@ async def client(test_settings: Settings) -> AsyncClient:  # type: ignore[misc]
     Yields:
         An :class:`httpx.AsyncClient` with ``base_url="http://test"``.
     """
+    from unittest.mock import MagicMock
+
     app = create_app(settings=test_settings)
 
     # Ensure fail2ban is reported as online for tests (mock socket is not
     # actually connected so we need to set the cached status manually).
     app.state.server_status = ServerStatus(online=True)
 
+    # Mock scheduler for health check tests (lifespan not run in ASGITransport tests)
+    mock_scheduler = MagicMock()
+    mock_scheduler.running = True
+    app.state.scheduler = mock_scheduler
+
     # Bootstrap the database schema before making requests.  ASGITransport
     # does not run the application lifespan, so we create the test SQLite file
     # directly rather than relying on startup logic.