fix: adapt batch dedup to NULL pattern_id — group by merge_group_hint

All Pass 0b controls have pattern_id=NULL. Rewritten to:
- Phase 1: Group by merge_group_hint (action:object:trigger), 52k groups
- Phase 2: Cross-group embedding search for semantically similar masters
- Qdrant search uses unfiltered cross-regulation endpoint
- API param changed: pattern_id → hint_filter

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend-compliance/compliance/api/crosswalk_routes.py

@@ -776,12 +776,12 @@ _batch_dedup_runner = None
 @router.post("/migrate/batch-dedup", response_model=MigrationResponse)
 async def migrate_batch_dedup(
     dry_run: bool = Query(False, description="Preview mode — no DB changes"),
-    pattern_id: Optional[str] = Query(None, description="Only process this pattern"),
+    hint_filter: Optional[str] = Query(None, description="Only process hints matching this prefix"),
 ):
     """Batch dedup: reduce ~85k Pass 0b controls to ~18-25k masters.
 
-    Groups controls by pattern_id + merge_group_hint, picks the best
-    quality master, and links duplicates via control_parent_links.
+    Phase 1: Groups by merge_group_hint, picks best quality master, links rest.
+    Phase 2: Cross-group embedding search for semantically similar masters.
     """
     global _batch_dedup_runner
     from compliance.services.batch_dedup_runner import BatchDedupRunner
@@ -790,7 +790,7 @@ async def migrate_batch_dedup(
     try:
         runner = BatchDedupRunner(db=db)
         _batch_dedup_runner = runner
-        stats = await runner.run(dry_run=dry_run, pattern_filter=pattern_id)
+        stats = await runner.run(dry_run=dry_run, hint_filter=hint_filter)
         return MigrationResponse(status="completed", stats=stats)
     except Exception as e:
         logger.error("Batch dedup failed: %s", e)