Add notification history feature with bell icon and history page

- Add notification_history database table for logging all triggered notifications
- Create API endpoints for fetching recent and historical notifications
- Add NotificationBell component in navbar with badge showing recent count
- Dropdown shows 5 most recent notifications with links to products
- Create full NotificationHistory page with filtering by notification type
- Log notifications when sent: price drops, target prices, back-in-stock
- Track which channels (telegram, discord, pushover, ntfy) received each notification
- Update sendNotifications to return which channels succeeded
- Bump version to 1.0.3

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -7,6 +7,7 @@ import Register from './pages/Register';
 import Dashboard from './pages/Dashboard';
 import ProductDetail from './pages/ProductDetail';
 import Settings from './pages/Settings';
+import NotificationHistory from './pages/NotificationHistory';
 
 function ThemeInitializer({ children }: { children: React.ReactNode }) {
   useEffect(() => {
@@ -111,6 +112,14 @@ function AppRoutes() {
           </ProtectedRoute>
         }
       />
+      <Route
+        path="/notifications"
+        element={
+          <ProtectedRoute>
+            <NotificationHistory />
+          </ProtectedRoute>
+        }
+      />
       <Route path="*" element={<Navigate to="/" replace />} />
     </Routes>
   );