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/api/deps.py

@@ -1,3 +1,4 @@
+import uuid
 from collections.abc import Generator
 from typing import Annotated, Any
 
@@ -157,7 +158,16 @@ def _user_for(session: Session, token_data: TokenPayload) -> User | None:
         return session.exec(
             select(User).where(User.portal_sub == token_data.portal_sub)
         ).first()
-    return session.get(User, token_data.sub) if token_data.sub else None
+    if not token_data.sub:
+        return None
+    try:
+        # The subject is a string on the wire. Postgres cast it on the way in;
+        # nothing else does, so parse it here rather than hand a driver a
+        # string where it wants a UUID.
+        user_id = uuid.UUID(token_data.sub)
+    except ValueError:
+        return None
+    return session.get(User, user_id)
 
 
 def user_from_token(