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

backend/app/api/ml_admin.py

@@ -21,6 +21,7 @@ _EDITABLE = (
     "head_auto_apply_precision",
     "head_auto_apply_enabled",
     "head_auto_apply_min_positives",
+    "ccip_match_threshold",
 )
 
 
@@ -48,6 +49,7 @@ async def get_settings():
             "head_auto_apply_precision": s.head_auto_apply_precision,
             "head_auto_apply_enabled": s.head_auto_apply_enabled,
             "head_auto_apply_min_positives": s.head_auto_apply_min_positives,
+            "ccip_match_threshold": s.ccip_match_threshold,
         }
     )
 
@@ -115,6 +117,8 @@ def _validate(p: dict) -> str | None:
         return "head_auto_apply_precision must be between 0.5 and 0.999"
     if int(p["head_auto_apply_min_positives"]) < 1:
         return "head_auto_apply_min_positives must be >= 1"
+    if not (0.5 <= float(p["ccip_match_threshold"]) <= 0.999):
+        return "ccip_match_threshold must be between 0.5 and 0.999"
     return None