feat: Data Management tab — purge endpoints for all major data stores

Backend — new DELETE /purge-all endpoints:
  - /api/logs/purge-all          → truncate system_logs (immediate, no 30d wait)
  - /api/reports/purge-all       → cycle_reports + ai_reports
  - /api/portfolio/purge-all     → portfolio + trade_entry_prices
  - /api/analytics/purge-all     → pattern_score_history, regime_clusters,
                                    pattern_embeddings, cycle_runs,
                                    macro_regime_history, geo_alert_history
  - /api/var/purge-all           → var_snapshots + pnl_snapshots
  - /api/pattern-lab/purge-all   → backtest_lab_runs

Frontend — Config.tsx: new 'Data Management' tab
  - PurgeButton component with inline double-confirm (click Purge → confirm)
  - Shows table names affected + row count deleted
  - 6 purge actions: Logs, AI Reports, Portfolio, Analytics, VaR, Pattern Lab

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

backend/routers/var.py

@@ -109,6 +109,19 @@ class SchedulerConfig(BaseModel):
     pnl_hours: float | None = None
 
 
+@router.delete("/purge-all")
+def purge_all_var():
+    """Delete all VaR snapshots and PnL snapshots."""
+    from services.database import get_conn
+    conn = get_conn()
+    conn.execute("DELETE FROM var_snapshots")
+    conn.execute("DELETE FROM pnl_snapshots")
+    n = conn.total_changes
+    conn.commit()
+    conn.close()
+    return {"deleted": n}
+
+
 @router.post("/scheduler/config")
 def scheduler_config(cfg: SchedulerConfig):
     if cfg.var_enabled is not None: