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/flow/runs.py

@@ -39,7 +39,7 @@ from datetime import datetime, timedelta, timezone
 from typing import Any
 
 from sqlalchemy import update
-from sqlalchemy.dialects.postgresql import insert as pg_insert
+from sqlalchemy.dialects.sqlite import insert as upsert
 from sqlmodel import Session, col, select
 
 from fluksio.core.db import engine as db_engine
@@ -258,9 +258,7 @@ class MetricSink:
     def _write(self, rows: list[RunMetric]) -> None:
         try:
             with Session(db_engine) as session:
-                statement = pg_insert(RunMetric).values(
-                    [row.model_dump() for row in rows]
-                )
+                statement = upsert(RunMetric).values([row.model_dump() for row in rows])
                 session.exec(
                     statement.on_conflict_do_update(
                         index_elements=["run_id", "name", "step"],