feat: Find Similar + Merge in Pattern Library

Backend (patterns.py):
- POST /api/patterns/find-similar — GPT-4o-mini compares a library pattern
  against all others; returns merge_as_instance | counter_scenario | new_pattern
- POST /api/patterns/merge — full transactional merge: remaps pattern_id in
  pattern_score_history, trade_entry_prices, ai_reasoning_traces, ai_call_logs,
  skipped_trades; unions historical_instances (dedup); sums backtest counters;
  deletes the discarded pattern

Frontend (PatternExplorer.tsx + useApi.ts):
- ScanSearch button on each non-builtin card triggers find-similar
- Inline result panel: duplicate → merge CTA with confirmation + destructive warning
  counter_scenario → apply regime_tag CTA; new_pattern → green "unique" badge
- useFindSimilarPattern + useMergePatterns hooks invalidate all-patterns on success

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

frontend/src/hooks/useApi.ts

@@ -363,6 +363,33 @@ export const useTogglePattern = () => {
   })
 }
 
+export interface SimilarityResult {
+  recommendation: 'merge_as_instance' | 'counter_scenario' | 'new_pattern'
+  match_id: string | null
+  match_name: string | null
+  confidence: number
+  reasoning: string
+  suggested_regime_tag: string
+}
+
+export const useFindSimilarPattern = () =>
+  useMutation({
+    mutationFn: (pattern_id: string) =>
+      api.post('/patterns/find-similar', { pattern_id }).then(r => r.data as SimilarityResult),
+  })
+
+export const useMergePatterns = () => {
+  const qc = useQueryClient()
+  return useMutation({
+    mutationFn: (body: { keep_id: string; discard_id: string }) =>
+      api.post('/patterns/merge', body).then(r => r.data),
+    onSuccess: () => {
+      qc.invalidateQueries({ queryKey: ['all-patterns'] })
+      qc.invalidateQueries({ queryKey: ['last-scores'] })
+    },
+  })
+}
+
 // ── Auto-Cycle ───────────────────────────────────────────────────────────────
 
 export const useCycleStatus = () =>