refactor(backend): extract auto-heal, notifications, console, sso-config routers (phase 4c-1) (#739)

Move four small Round C route groups out of index.ts:
- /api/auto-heal/* -> routes/autoHeal.ts
- /api/notifications/*, /api/notification-routes/* -> routes/notifications.ts
- /api/system/console-token -> routes/console.ts
- /api/sso/config/* -> routes/ssoConfig.ts

Share NOTIFICATION_CHANNEL_TYPES, cleanStackPatterns, and validateHttpsUrl
via a new helpers/notificationChannels.ts so agents.ts and notifications.ts
consume the same allowlist and URL validator.

Handlers moved verbatim. Middleware chains and response shapes preserved.
index.ts drops from 3231 to 2739 lines.

backend/src/helpers/notificationChannels.ts

@@ -0,0 +1,11 @@
+export const NOTIFICATION_CHANNEL_TYPES = ['discord', 'slack', 'webhook'] as const;
+export type NotificationChannelType = typeof NOTIFICATION_CHANNEL_TYPES[number];
+
+export const cleanStackPatterns = (patterns: string[]): string[] =>
+  [...new Set(patterns.map(p => p.trim()).filter(Boolean))];
+
+export function validateHttpsUrl(value: unknown): string | null {
+  if (!value || typeof value !== 'string' || !value.startsWith('https://')) return 'must be a valid HTTPS URL';
+  try { new URL(value); } catch { return 'is not a valid URL'; }
+  return null;
+}