feat(gpu): HTTP job API + token auth + backfill — the agent's server side (#114 slice 3b)

The thin HTTP surface over the queue so the desktop agent stays HTTP-only:
- Agent endpoints (Authorization: Bearer <token>): POST /api/gpu/jobs/lease
  (returns jobs + image_url + mime + video frame cadence), /submit (stores
  regions via RegionService + closes the job; 409 on a stale lease), /heartbeat,
  /fail. Token validated against AppSetting (mirrors the extension-key pattern,
  constant-time compare).
- Admin (browser): GET/POST /api/gpu/token[/rotate] (generate + show the agent
  token), GET /api/gpu/status (queue counts), POST /api/gpu/backfill → dispatches
  enqueue_gpu_backfill.
- enqueue_gpu_backfill(task): one INSERT…SELECT enqueues a job per image lacking
  one for the task (scales to the full library; idempotent).

Agent flow: lease over HTTP → fetch pixels via the normal FC image URL → compute
on the GPU → submit. Redis/Postgres never exposed.

Tests: bearer required (+ wrong-token 401), lease→submit round-trip (region+CCIP
vector stored, job done via /status), stale-lease 409, backfill enqueue +
idempotency.

NEXT: the agent container + control UI, then the CCIP detector/embedder + matcher.

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

backend/app/api/__init__.py

@@ -25,6 +25,7 @@ def all_blueprints() -> list[Blueprint]:
     from .downloads import downloads_bp
     from .extension import extension_bp
     from .gallery import gallery_bp
+    from .gpu import gpu_bp
     from .heads import heads_bp
     from .import_admin import import_admin_bp
     from .ml_admin import ml_admin_bp
@@ -60,6 +61,7 @@ def all_blueprints() -> list[Blueprint]:
         aliases_bp,
         tag_eval_bp,
         heads_bp,
+        gpu_bp,
         ml_admin_bp,
         thumbnails_bp,
         sources_bp,