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>

docker/compose.ci.yml

@@ -18,11 +18,6 @@ services:
 
   db:
     container_name: !reset null
-    # The one service a job may need from the host: test-backend.yml runs pytest
-    # on the runner. Host side left to Docker, so concurrent jobs get a port
-    # each — `docker compose port db 5432` says which.
-    ports: !override
-      - "5432"
 
   redis:
     container_name: !reset null
@@ -34,9 +29,6 @@ services:
   mailcatcher:
     ports: !reset []
 
-  prestart:
-    container_name: !reset null
-
   backend:
     container_name: !reset null
     ports: !reset []