feat: system logs page + dynamic IV watchlist with auto-add from cycle

Backend:
- DB: add system_logs table (level/source/cycle_id/ticker/message) and
  iv_watchlist table (ticker/added_by/is_active); seed builtin 18 tickers
- DBLogHandler attached at startup — all WARNING+ logs auto-persist to DB
- log_system_event() helper for structured manual events
- New router /api/logs: GET with filters (level, source, cycle_id, ticker,
  date range), GET /sources, GET /cycles for dropdowns, DELETE /clear
- iv_watchlist now read from DB instead of hardcoded constant; options_vol
  watchlist/refresh/bootstrap endpoints all use get_watchlist_tickers()
- New endpoints: POST/DELETE /options-vol/watchlist-tickers/{ticker} to
  add/remove tickers; adding triggers background 1-year bootstrap
- auto_cycle: after log_trade_entries(), auto-detect new underlying proxies
  not yet in watchlist, add them and bootstrap their IV history

Frontend:
- New page SystemLogs (/logs): log table with level/source/cycle/ticker/date
  filters, color-coded rows, expandable JSON details, auto-refresh 30s
- Options Lab: WatchlistManager section — add ticker input, chip list with
  builtin/auto/manual color coding, remove button for non-builtins
- Sidebar: Logs Système nav link (ScrollText icon)
- useApi: useSystemLogs, useLogSources, useLogCycles, useClearLogs,
  useWatchlistTickers, useAddWatchlistTicker, useRemoveWatchlistTicker

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

frontend/src/App.tsx

@@ -16,6 +16,7 @@ import Config from './pages/Config'
 import Analytics from './pages/Analytics'
 import AnalyticsAdvanced from './pages/AnalyticsAdvanced'
 import RiskDashboard from './pages/RiskDashboard'
+import SystemLogs from './pages/SystemLogs'
 import { useCycleWatcher } from './hooks/useApi'
 
 function GlobalWatcher() {
@@ -47,6 +48,7 @@ export default function App() {
             <Route path="/analytics" element={<Analytics />} />
             <Route path="/analytics-advanced" element={<AnalyticsAdvanced />} />
             <Route path="/risk" element={<RiskDashboard />} />
+            <Route path="/logs" element={<SystemLogs />} />
           </Routes>
         </main>
       </div>