feat(gpu): video-ready regions + the HTTP GPU-job queue engine (#114 slice 3)

Answers "how are videos/all media handled by the GPU worker": a job is per ITEM,
but the agent fans a VIDEO into per-frame instances (ffmpeg in the agent, the
existing cadence), each stored with a timestamp — so a video becomes a BAG of
frame embeddings (fixes the mean-embedding muddle) instead of one washed-out
vector. Stills → frame_time NULL; animated GIF/WebP treated like short video.

- image_region.frame_time (migration 0061, not yet deployed so folded in): the
  source frame's seconds for video/animated media; NULL for stills. RegionService
  passes it through. A whole frame is just kind='frame'.
- gpu_job + GpuJobService (migration 0062): the durable work list that keeps the
  desktop agent HTTP-only — enqueue (dedupes (image,task)) / lease (FOR UPDATE
  SKIP LOCKED, re-claims expired leases so the queue self-heals) / heartbeat /
  complete / fail (re-queues until MAX_ATTEMPTS then 'error'). The server enqueues;
  the agent leases+submits over the web API; Redis/Postgres stay private.

Tests: enqueue dedupe, lease-then-skip-when-held, expired-lease reclaim, scoped
heartbeat, complete, fail-requeue-then-error. region test now covers frame_time.

NEXT: the thin HTTP API (lease/submit/heartbeat) + bearer-token auth, then the
agent container + control UI.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

backend/app/models/image_region.py

@@ -30,8 +30,13 @@ class ImageRegion(Base):
     image_record_id: Mapped[int] = mapped_column(
         ForeignKey("image_record.id", ondelete="CASCADE"), index=True
     )
-    # 'face' | 'figure' (→ CCIP character id) | 'concept' (→ SigLIP head bag).
+    # 'frame' (a whole video frame → SigLIP bag) | 'face' | 'figure' (→ CCIP
+    # character id) | 'concept' (→ SigLIP head bag).
     kind: Mapped[str] = mapped_column(String(16), nullable=False)
+    # For video/animated media: the source frame's timestamp in SECONDS. NULL for
+    # static images. Lets a video be a BAG of per-frame instances (fixes the
+    # mean-embedding muddle) + grounds a tag to "appears at 0:42".
+    frame_time: Mapped[float | None] = mapped_column(Float, nullable=True)
     # Normalized bbox in [0,1]: top-left (rx, ry) + size (rw, rh). Named rx/ry/…
     # rather than x/y/by to dodge SQL keyword ambiguity ('by').
     rx: Mapped[float] = mapped_column(Float, nullable=False)