feat(inference): S7 Call 1/Call 2 + prompt revision REST + test-inference + Generate text UI

- backend/inference.py: prompt seed (rev 0) + append-only revision REST
  (POST /prompts/{call}/revisions, /activate, GET; METADATA-105/106). Call 1 vision
  classification (1024px processed photos + synced taxonomy, 0.7 confidence gate,
  METADATA-35/40/45), Call 2 long description (user-triggered, METADATA-55/60/65),
  inference_log per call (METADATA-66), inference-off server_description (METADATA-70),
  test-inference probe (SETTINGS-45). Stdlib urllib, no new deps.
- frontend: create/inference/GenerateText.svelte (control visible only when content
  enabled + endpoint reachable, METADATA-110; Call 2 + Call 1 buttons) + dev toggle.
- tests: 6 inference tests (seed+revision, call1 gate, call2 stores desc, off->409,
  server desc, test-inference unconfigured).

Verified: pytest 54 passed; gates exit 0; npm build ok.

backend/app.py

@@ -271,5 +271,8 @@ app.include_router(review_router)
 from .publish import router as publish_router
 app.include_router(publish_router)
 
+from .inference import router as inference_router
+app.include_router(inference_router)
+
 # expose for tests
 __all__ = ["app", "__version__"]