feat: add purge-all for Pattern Library + clarify Trade Ideas in Data Management

Backend: DELETE /api/patterns/purge-all — removes all custom + backtested patterns
(source != 'builtin'), leaves Pattern Lab run history intact.

Frontend Config > Data Management:
- New PurgeButton for Pattern Library (custom_patterns table)
- Updated note: Trade Ideas are computed live from the Pattern Library (no separate table),
  so purging patterns resets trade idea generation on next cycle.

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

backend/routers/patterns.py

@@ -61,6 +61,18 @@ def update_pattern(pat_id: str, req: PatternRequest):
     return {"id": pat_id, "status": "updated"}
 
 
+@router.delete("/purge-all")
+def purge_all_patterns():
+    """Delete ALL custom/backtested patterns from the pattern library."""
+    from services.database import get_conn
+    conn = get_conn()
+    conn.execute("DELETE FROM custom_patterns WHERE source != 'builtin'")
+    n = conn.total_changes
+    conn.commit()
+    conn.close()
+    return {"deleted": n}
+
+
 @router.delete("/custom/{pat_id}")
 def delete_pattern(pat_id: str):
     delete_custom_pattern(pat_id)