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_oauth.py

@@ -3,12 +3,15 @@
 import base64
 import hashlib
 import secrets
+import uuid
 from urllib.parse import parse_qs, urlparse
 
 import pytest
 from fastapi.testclient import TestClient
+from sqlmodel import Session, select
 
 from fluksio.core.config import settings
+from fluksio.models import OAuthRefreshToken
 
 PREFIX = f"{settings.API_V1_STR}/oauth"
 REDIRECT = "http://127.0.0.1:41234/callback"
@@ -235,6 +238,7 @@ def test_everything_is_refused_while_mcp_is_off(
 
 def test_one_agent_can_be_revoked_without_touching_the_others(
     client: TestClient,
+    db: Session,
     superuser_token_headers: dict[str, str],
     normal_user_token_headers: dict[str, str],
 ) -> None:
@@ -272,3 +276,13 @@ def test_one_agent_can_be_revoked_without_touching_the_others(
         data={"grant_type": "refresh_token", "refresh_token": tokens["refresh_token"]},
     )
     assert refreshed.status_code == 400
+
+    # Went, rather than merely stopped working: nothing here deletes those rows
+    # itself, so an orphan is a foreign key the database is not enforcing —
+    # which SQLite does not do unless it is asked (`PRAGMA foreign_keys`).
+    left = db.exec(
+        select(OAuthRefreshToken).where(
+            OAuthRefreshToken.client_id == uuid.UUID(client_id)
+        )
+    ).all()
+    assert left == []