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/test_db.py

@@ -0,0 +1,33 @@
+"""What the database has to keep true whatever dialect is under it."""
+
+from datetime import datetime, timedelta, timezone
+
+from sqlmodel import Session, select
+
+from fluksio.core.db import engine
+from fluksio.models import EngineEvent
+
+# The `db` fixture is session-scoped and autouse, so the schema is already there.
+
+
+def test_a_stored_instant_comes_back_aware_and_in_utc() -> None:
+    """SQLite stores no offset, so the column type has to put one back.
+
+    Without it a timestamp read from the database is naive: comparing it to
+    `datetime.now(timezone.utc)` raises, and serialising it hands the frontend
+    a time with no zone, which it reads as local.
+    """
+    stamp = datetime(2026, 8, 21, 15, 30, tzinfo=timezone(timedelta(hours=2)))
+    with Session(engine) as session:
+        session.add(
+            EngineEvent(ts=stamp, type="node_error", flow="tz", node="n", detail="")
+        )
+        session.commit()
+
+    with Session(engine) as session:
+        stored = session.exec(select(EngineEvent).where(EngineEvent.flow == "tz")).one()
+        assert stored.ts.tzinfo is not None
+        assert stored.ts.utcoffset() == timedelta(0)
+        assert stored.ts == stamp
+        # And it still compares against an aware "now" rather than raising.
+        assert stored.ts < datetime.now(timezone.utc)