feat(ccip): refresh task + beat + retrain hook for character prototypes (#1317, m138 step 3)

- refresh_character_prototypes celery task wraps the incremental builder (sync
  ml worker); returns skipped / rebuilt=N removed=N.
- Beat: every ~15 min (cheap global-gate no-op when idle) + a nightly full=True
  reconcile as belt-and-suspenders.
- train_heads enqueues it on success, so the Retrain button AND the nightly head
  retrain refresh CCIP on the SAME trigger — unified lifecycle, as asked.
The initial (cold) full build loads the whole reference set once in the
background, never on a /suggestions request.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/celery_app.py

@@ -107,6 +107,15 @@ def make_celery() -> Celery:
                 "task": "backend.app.tasks.ml.scheduled_train_heads",
                 "schedule": 86400.0,  # passive cadence; manual retrain stays available
             },
+            "refresh-character-prototypes": {
+                "task": "backend.app.tasks.ml.refresh_character_prototypes",
+                "schedule": 900.0,  # ~15 min; cheap global-gate no-op when idle (#1317)
+            },
+            "reconcile-character-prototypes-nightly": {
+                "task": "backend.app.tasks.ml.refresh_character_prototypes",
+                "schedule": 86400.0,  # nightly FULL reconcile (belt-and-suspenders)
+                "args": (True,),      # full=True
+            },
             "apply-head-tags-daily": {
                 "task": "backend.app.tasks.ml.scheduled_apply_head_tags",
                 "schedule": 86400.0,  # no-op unless head_auto_apply_enabled