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/api/endpoints/flow_proposals.py

@@ -18,6 +18,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy.orm import selectinload
 
 from app.core.rate_limit import limiter
+from app.services.notification_service import notify
 from app.api.deps import get_current_active_user, get_db, require_engineer_or_admin, require_team_admin
 from app.models.user import User
 from app.models.tree import Tree
@@ -262,6 +263,13 @@ async def review_proposal(
     elif data.action == "dismiss":
         proposal.status = "dismissed"
 
+    if data.action == "approve":
+        await notify("proposal.approved", proposal.account_id, {
+            "title": proposal.title,
+            "reviewer_name": current_user.display_name if hasattr(current_user, 'display_name') else current_user.email,
+            "link": "/review-queue",
+        }, db, target_user_ids=[proposal.created_by_id] if proposal.created_by_id else None)
+
     await db.commit()
 
     return FlowProposalDetail.model_validate(proposal)