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>

frontend/src/pages/account/IntegrationsPage.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useState } from 'react'
-import { Plug, CheckCircle2, AlertCircle, Loader2, Pencil, Trash2, Shield, History, Ticket, Users, Zap, Save } from 'lucide-react'
+import { Plug, CheckCircle2, AlertCircle, Loader2, Pencil, Trash2, Shield, History, Ticket, Users, Zap, Save, Bell } from 'lucide-react'
+import { NotificationSettings } from '@/components/account/NotificationSettings'
 import { analytics } from '@/lib/analytics'
 import { EmptyState } from '@/components/common/EmptyState'
 import { IntegrationIllustration } from '@/components/common/EmptyStateIllustrations'
@@ -41,7 +42,7 @@ const emptyForm: ConnectionForm = {
   private_key: '',
 }
 
-type Tab = 'connection' | 'member-mapping' | 'post-history' | 'flowpilot-settings'
+type Tab = 'connection' | 'member-mapping' | 'post-history' | 'flowpilot-settings' | 'notifications'
 
 export function IntegrationsPage() {
   const [activeTab, setActiveTab] = useState<Tab>('connection')
@@ -237,6 +238,7 @@ export function IntegrationsPage() {
             { id: 'member-mapping' as Tab, label: 'Member Mapping', icon: Users },
             { id: 'post-history' as Tab, label: 'Post History', icon: History },
             { id: 'flowpilot-settings' as Tab, label: 'FlowPilot', icon: Zap },
+            { id: 'notifications' as Tab, label: 'Notifications', icon: Bell },
           ]).map(({ id, label, icon: Icon }) => (
             <button
               key={id}
@@ -555,6 +557,11 @@ export function IntegrationsPage() {
         {activeTab === 'flowpilot-settings' && (
           <FlowPilotSettingsTab connection={connection} />
         )}
+
+        {/* Notifications Tab */}
+        {activeTab === 'notifications' && (
+          <NotificationSettings />
+        )}
       </div>
     </>
   )