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/ml_settings.py

@@ -155,6 +155,18 @@ class MLSettings(Base):
     detector_dedupe_iou: Mapped[float] = mapped_column(
         Float, nullable=False, default=0.85
     )
+    # -- CCIP character prototypes (#1317) ---------------------------------
+    # The per-character reference set is precomputed + refreshed INCREMENTALLY
+    # (services.ml.character_prototypes) instead of rebuilt on the request path.
+    # ccip_ref_signature is the cheap GLOBAL gate — when it's unchanged the
+    # refresh no-ops; ccip_prototype_cap bounds the reference vectors kept per
+    # character so MATCH cost doesn't grow with a character's popularity.
+    ccip_ref_signature: Mapped[str | None] = mapped_column(
+        String(128), nullable=True
+    )
+    ccip_prototype_cap: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=64
+    )
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()
     )