mAi: #8 - imagen.jobs queue + worker subcommand (flexsiebels write path)

Async write path for the flexsiebels owner-mode UI: flexsiebels INSERTs into
imagen.jobs, the worker on mRiver claims pending rows via LISTEN/NOTIFY +
5s safety poll, runs the same generate pipeline imagen generate uses, and
writes the result through internal/cloud into imagen.images.

- Schema migration imagen_jobs_init: table + status CHECK + two indexes +
  owner-scoped RLS + grants + AFTER INSERT trigger publishing on the
  imagen_jobs channel via pg_notify.
- internal/worker: DB-agnostic loop over a Queue interface. Drains the
  whole pending backlog on each wake. Job-scoped contexts are derived
  from Background so SIGTERM lets the in-flight generation finish (no
  half-state). ResetStaleRunning at startup unsticks rows left over from
  a previous crash. Eight unit tests cover the done / failed / missing-id /
  drain / NOTIFY-wake / shutdown / transient-error paths against a fake
  queue (no real Postgres in CI).
- cmd/imagen/worker.go: pgx-backed Queue (one dedicated conn for LISTEN +
  UPDATE), plus the workerPipeline that reuses buildBackend +
  attachUsageSink + prompt.Apply + buildWriter + maybeCloudSync. The
  per-job owner_user_id overrides the env-level fallback so each row in
  imagen.images is attributed correctly.
- maybeCloudSync now returns (*cloud.SyncResult, error) so the worker can
  link imagen.jobs.image_id to the inserted imagen.images row. The CLI
  generate path keeps printing its stderr summary unchanged.
- scripts/imagen-worker.service + .env.example for the systemd --user unit
  on mRiver. EnvironmentFile lives in ~/.dotfiles and is never committed.
- docs/setup-worker-mriver.md walks through installation + the spec's
  SQL-INSERT smoke; docs/architecture.md grows an "async write path"
  section.
- worker_integration_test.go (env-guarded by IMAGEN_WORKER_INTEGRATION=1)
  drives one real job through the full pipeline against msupabase using
  the mock backend, then verifies imagen.images + Storage object landed
  and the row flipped to done with image_id linked. Verified end-to-end:
  pickup latency ~7ms, total 74ms, failure path captures error text.

go.mod

@@ -1,5 +1,16 @@
 module mgit.msbls.de/m/ImaGen
 
-go 1.24
+go 1.25.0
 
-require gopkg.in/yaml.v3 v3.0.1
+require (
+	github.com/jackc/pgx/v5 v5.9.2
+	gopkg.in/yaml.v3 v3.0.1
+)
+
+require (
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
+	github.com/kr/text v0.2.0 // indirect
+	github.com/rogpeppe/go-internal v1.14.1 // indirect
+	golang.org/x/text v0.29.0 // indirect
+)