feat(ccip): tunable match threshold, default 0.85 (#114)

Live data showed the v1 flat 0.75 cosine over-fired — ~64% of matched images got
3-10 character guesses dominated by the most-referenced characters (a 27-ref
character clears a low bar on many images). A sweep showed 0.85 collapses the
noise (noisy multi-matches 47→3) while keeping the confident single-character
matches.

- ml_settings.ccip_match_threshold (migration 0063, default 0.85); match_image
  reads it (override still accepted). DEFAULT_SIM_THRESHOLD fallback 0.75→0.85.
- Exposed in GET/PATCH /api/ml/settings (validated 0.5–0.999).
- Slider in the GPU agent card ("Character-match strictness") — tune live, no
  redeploy, same observe-and-tune loop as auto-apply.

Test: a ~0.9-cosine figure matches at 0.85, dropped at 0.95.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

alembic/versions/0063_ccip_match_threshold.py

@@ -0,0 +1,33 @@
+"""ml_settings.ccip_match_threshold — tunable CCIP character-match cut (#114)
+
+The v1 matcher used a flat 0.75 cosine; live data showed that over-fires (a
+high-reference character matched a scatter of images). 0.85 keeps the confident
+single-character matches and drops the noise. Tunable from the GPU agent card.
+
+Revision ID: 0063
+Revises: 0062
+Create Date: 2026-06-29
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0063"
+down_revision: Union[str, None] = "0062"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "ml_settings",
+        sa.Column(
+            "ccip_match_threshold", sa.Float(), nullable=False,
+            server_default="0.85",
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("ml_settings", "ccip_match_threshold")