feat(ccip): schema for precomputed incremental character prototypes (#1317, m138 step 1)

Foundation for making CCIP character references a precomputed, INCREMENTAL
artifact instead of a request-path rebuild (kills the per-accept ~4s suggestions
stall; cost will scale with change, not library size):

- character_prototype: a character's reference CCIP vectors, capped to
  MLSettings.ccip_prototype_cap so match cost doesn't grow with popularity.
- ccip_prototype_state: per-character fingerprint (ref count + max region id) +
  updated_at → drives per-character incremental rebuilds and the matcher cache's
  reload-only-what-advanced.
- MLSettings.ccip_ref_signature (cheap global change gate) + ccip_prototype_cap.

Migration 0079. Schema + models only — the builder service, refresh task/beat,
and matcher rewrite land in the following steps.

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

backend/app/models/__init__.py

@@ -5,6 +5,7 @@ from .artist import Artist
 from .artist_visit import ArtistVisit
 from .backup_run import BackupRun
 from .base import Base
+from .character_prototype import CcipPrototypeState, CharacterPrototype
 from .credential import Credential
 from .download_event import DownloadEvent
 from .external_link import ExternalLink
@@ -79,6 +80,8 @@ __all__ = [
     "HeadTrainingRun",
     "TagAlias",
     "TagHead",
+    "CharacterPrototype",
+    "CcipPrototypeState",
     "TagPositiveConfirmation",
     "TagSuggestionRejection",
     "TaskRun",