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>

.gitea/workflows/test-compose.yml

@@ -47,8 +47,9 @@ jobs:
       # breaks silently.
       - run: docker compose build backend frontend
       - run: docker compose down -v --remove-orphans
-      # db and prestart come along as depends_on; --wait blocks on the backend
-      # healthcheck in docker/compose.yml.
+      # Redis comes along as a depends_on; --wait blocks on the backend
+      # healthcheck in docker/compose.yml, which is also what proves the
+      # schema was created before anything served a request.
       - run: docker compose up -d --wait backend frontend
       # Nothing is published on the host (compose.ci.yml), so the checks run
       # from inside the backend container against the project network.