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/metrics.py

@@ -19,7 +19,7 @@ from datetime import datetime, timedelta, timezone
 from typing import Any
 
 from sqlalchemy import delete, func, update
-from sqlalchemy.dialects.postgresql import insert
+from sqlalchemy.dialects.sqlite import insert
 from sqlmodel import Session, col
 
 from fluksio.core.config import settings
@@ -354,9 +354,9 @@ class MetricsCollector:
                             for name in SUMMED
                         }
                         | {
-                            name: func.greatest(
-                                col(getattr(MetricBucket, name)), new[name]
-                            )
+                            # `max` of two values, not the aggregate: SQLite's
+                            # scalar form, which is what `greatest` is elsewhere.
+                            name: func.max(col(getattr(MetricBucket, name)), new[name])
                             for name in MAXIMA
                         },
                     )