Keep the engine's state in SQLite, not Postgres

One process owns this database — the image has run a single uvicorn
worker for that reason since the four-engines bug — so a file beside the
flows is the honest shape for it, and it is what lets `fluksio serve`
need no infrastructure at all. Live values, node execution and the work
queue never came here anyway; what does is a rollup a minute at a time,
a row per cascade and the run history, and WAL keeps the readers going
while that one writer works.

DATA_DIR is now the one setting that moves everything an installation
keeps; the rest derive from it and the images still spell theirs out.
The schema is prepared in-process at startup, so the prestart service is
gone, and the ten Postgres-only revisions collapse into one portable
baseline.

Three things only worked because psycopg was casting for us: a token's
subject arriving as a string where the column is a UUID, `greatest`, and
`date_bin`. The timestamps needed a column type of their own — SQLite
stores no offset, and a naive datetime read back either raises against an
aware `now` or serialises as local time.

Postgres stays in the stack only for Umami, behind the analytics profile.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/fluksio/main.py

@@ -16,6 +16,8 @@ from fluksio.api.routes.alerts import read_config as read_alerts_config
 from fluksio.cloud import config as cloud_config
 from fluksio.core import security
 from fluksio.core.config import settings
+from fluksio.core.db import engine as db_engine
+from fluksio.core.db import prepare
 from fluksio.flow import logs, modules
 from fluksio.flow.alerts import AlertManager
 from fluksio.flow.artifacts import ArtifactStore
@@ -88,6 +90,10 @@ def _mcp_sessions() -> AbstractAsyncContextManager[None]:
 @asynccontextmanager
 async def lifespan(app: FastAPI) -> AsyncIterator[None]:
     """Start the flow engine alongside the API."""
+    # The schema and the first superuser, before anything reads either. It is
+    # idempotent, so a deployment that ran this from its own prestart step
+    # pays a version check for it and nothing else.
+    await run_in_threadpool(prepare, db_engine)
     event_bus.bind(asyncio.get_running_loop())
     # Node code is user code, and `print` is how it says things.
     logs.install()