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>

README.md

@@ -10,7 +10,7 @@ repo holds the FastAPI backend, the flow engine, and the dashboard SPA. It is se
 ## Layout
 
 ```text
-backend/        FastAPI + SQLModel + Alembic + Postgres
+backend/        FastAPI + SQLModel + Alembic + SQLite
   fluksio/flow/ the flow engine (nodes, pipeline, state backends, controller)
 frontend/       React 19 + TanStack Router + Tailwind 4 + shadcn/ui
 docker/         compose.yml → compose.dev.yml → compose.local.yml (+ compose.traefik.yml)
@@ -23,7 +23,7 @@ Normally driven from the workspace root (`make init` once, then `make dev`). Sta
 
 ```sh
 make install       # uv sync + bun install
-make dev-utils     # db, adminer, proxy, mailcatcher, prestart only
+make dev-utils     # proxy and mailcatcher only
 make dev-backend   # FastAPI on :8000, hot reload
 make dev-frontend  # Vite on :5173
 ```