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>

frontend/src/pages/Config.tsx

@@ -1629,15 +1629,23 @@ function DataManagementCard() {
 
       <PurgeButton
         label="Pattern Lab Runs"
-        description="Delete all Pattern Lab backtest runs (event presets + instrument scans)."
+        description="Delete all Pattern Lab backtest runs (event presets + instrument scans). Does not affect the Pattern Library."
         tables={['backtest_lab_runs']}
         endpoint="/api/pattern-lab/purge-all"
         color="orange"
       />
 
+      <PurgeButton
+        label="Pattern Library"
+        description="Delete all custom and backtested patterns from the library. Trade ideas will regenerate from scratch on the next cycle. Does not affect Pattern Lab run history."
+        tables={['custom_patterns']}
+        endpoint="/api/patterns/purge-all"
+        color="red"
+      />
+
       <div className="mt-2 p-3 border border-slate-700/40 rounded-lg bg-dark-700/30">
         <p className="text-xs text-slate-500">
-          <span className="font-semibold text-slate-400">Note:</span> Custom patterns, risk profiles, config, API keys, and the Knowledge Base are managed from their respective sections and are not affected by the purges above.
+          <span className="font-semibold text-slate-400">Note:</span> Trade Ideas are computed live from the Pattern Library — they have no separate table. Risk profiles, config, API keys, and the Knowledge Base are managed from their respective sections.
         </p>
       </div>
     </div>