feat(notifications): add Phase 4 Slice 2 — multi-channel notification system

Full notification infrastructure with in-app, email, Slack, and Teams channels:

Backend:
- NotificationConfig, NotificationLog, Notification models + migration
- Notification service with event routing, channel delivery, retry logic
- 9 API endpoints (config CRUD + in-app notifications)
- APScheduler retry job with exponential backoff (30s, 2m, 10m)
- Wired into escalation, proposal approval, and knowledge flywheel
- Pydantic event key validation, cross-tenant protection on recipients

Frontend:
- TypeScript types + API client for all notification endpoints
- NotificationsPanel: bell icon with unread badge, dropdown, mark-read
- NotificationSettings: channel config, event toggles, test, delete confirm
- Notifications tab on IntegrationsPage
- ARIA attributes, Escape handler, settings link on panel

Review fixes (13 issues resolved):
- notify() no longer commits/rolls back caller's transaction (critical)
- retry_failed_notifications returns count instead of None (critical)
- NotificationSettings moved inside dedicated tab (critical)
- target_user_ids scoped by account_id (security)
- Email loop collects all failures before raising
- Slack webhook validates response body
- events_enabled rejects unknown event keys
- link column widened to String(500)
- Dead code removed from _auto_reinforce
- Delete confirmation, ARIA, Escape key support

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/main.py

@@ -33,6 +33,7 @@ from app.core.rate_limit import limiter
 from app.api.router import api_router
 from app.core.scheduler import scheduler, load_all_schedules, _cleanup_expired_ai_conversations
 from app.services.retention_cleanup import cleanup_expired_chats
+from app.services.notification_service import retry_failed_notifications
 from app.core.service_account import ensure_service_account
 
 # Initialize logging configuration
@@ -61,6 +62,14 @@ async def archive_stale_ai_sessions():
         logger.info(f"[archive] Archived {result.rowcount} stale AI chat sessions")
 
 
+async def _process_notification_retries():
+    """Retry failed notification deliveries."""
+    async with async_session_maker() as db:
+        retried = await retry_failed_notifications(db)
+        if retried:
+            logger.info("Retried %d failed notifications", retried)
+
+
 def _configure_seed_module(mod: object, api_url: str, email: str, password: str) -> None:
     """Set globals on a seed script module."""
     mod.API_BASE_URL = api_url  # type: ignore[attr-defined]
@@ -201,6 +210,16 @@ async def lifespan(app: FastAPI):
         max_instances=1,
     )
 
+    # Notification retry (every minute)
+    scheduler.add_job(
+        _process_notification_retries,
+        trigger="interval",
+        minutes=1,
+        id="notification_retry",
+        replace_existing=True,
+        max_instances=1,
+    )
+
     # Auto-seed trees in background on PR environments
     seed_task = None
     if settings.SEED_ON_DEPLOY: