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

alembic/versions/0079_character_prototypes.py

@@ -0,0 +1,77 @@
+"""character prototype store (#1317) — precomputed, incremental CCIP references
+
+New tables character_prototype + ccip_prototype_state, plus MLSettings columns
+ccip_ref_signature (cheap global change gate) + ccip_prototype_cap (per-character
+reference cap). The reference set the CCIP matcher uses becomes a precomputed
+artifact refreshed incrementally off the request path. See milestone 138 /
+backend.app.services.ml.character_prototypes.
+
+Revision ID: 0079
+Revises: 0078
+Create Date: 2026-07-06
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+from pgvector.sqlalchemy import Vector
+
+revision: str = "0079"
+down_revision: Union[str, None] = "0078"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+# Matches models.image_region.CCIP_DIM (the CCIP figure-embedding width).
+_CCIP_DIM = 768
+
+
+def upgrade() -> None:
+    op.create_table(
+        "character_prototype",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column(
+            "tag_id", sa.Integer(),
+            sa.ForeignKey("tag.id", ondelete="CASCADE"), nullable=False,
+        ),
+        sa.Column("ccip_embedding", Vector(_CCIP_DIM), nullable=False),
+        sa.Column(
+            "region_id", sa.Integer(),
+            sa.ForeignKey("image_region.id", ondelete="SET NULL"), nullable=True,
+        ),
+    )
+    op.create_index(
+        "ix_character_prototype_tag_id", "character_prototype", ["tag_id"]
+    )
+    op.create_table(
+        "ccip_prototype_state",
+        sa.Column(
+            "tag_id", sa.Integer(),
+            sa.ForeignKey("tag.id", ondelete="CASCADE"), primary_key=True,
+        ),
+        sa.Column("fingerprint", sa.String(64), nullable=False),
+        sa.Column(
+            "updated_at", sa.DateTime(timezone=True), nullable=False,
+            server_default=sa.func.now(),
+        ),
+    )
+    op.add_column(
+        "ml_settings",
+        sa.Column("ccip_ref_signature", sa.String(128), nullable=True),
+    )
+    op.add_column(
+        "ml_settings",
+        sa.Column(
+            "ccip_prototype_cap", sa.Integer(), nullable=False,
+            server_default=sa.text("64"),
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("ml_settings", "ccip_prototype_cap")
+    op.drop_column("ml_settings", "ccip_ref_signature")
+    op.drop_table("ccip_prototype_state")
+    op.drop_index(
+        "ix_character_prototype_tag_id", table_name="character_prototype"
+    )
+    op.drop_table("character_prototype")