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/tests/api/routes/test_private.py

@@ -1,3 +1,5 @@
+import uuid
+
 import pytest
 from fastapi.testclient import TestClient
 from sqlmodel import Session, select
@@ -20,7 +22,8 @@ def test_create_user(client: TestClient, db: Session) -> None:
 
     data = r.json()
 
-    user = db.exec(select(User).where(User.id == data["id"])).first()
+    # The id comes back from JSON as a string, and the column is a UUID.
+    user = db.exec(select(User).where(User.id == uuid.UUID(data["id"]))).first()
 
     assert user
     assert user.email == "pollo@listo.com"