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/services/flowpilot_engine.py

@@ -17,6 +17,7 @@ from sqlalchemy.orm import selectinload
 
 from app.core.ai_provider import get_ai_provider
 from app.core.config import settings
+from app.services.notification_service import notify
 from app.models.ai_session import AISession
 from app.models.ai_session_step import AISessionStep
 from app.schemas.ai_session import (
@@ -497,6 +498,15 @@ async def escalate_session(
 
     await db.flush()
 
+    # Notify about escalation
+    await notify("session.escalated", session.account_id, {
+        "session_id": str(session_id),
+        "engineer_name": session.user.display_name if session.user else "Unknown",
+        "escalation_reason": request.escalation_reason,
+        "problem_summary": session.problem_summary or "N/A",
+        "link": f"/pilot/{session_id}",
+    }, db, target_user_ids=[request.escalated_to_id] if request.escalated_to_id else None)
+
     # Push documentation to PSA if ticket is linked
     psa_result = await _push_to_psa(session, user_id, db)