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/types/index.ts

@@ -93,3 +93,4 @@ export type {
 
 export * from './scripts'
 export * from './integrations'
+export * from './notification'

frontend/src/types/notification.ts

@@ -0,0 +1,52 @@
+export interface NotificationConfig {
+  id: string
+  channel: 'email' | 'slack_webhook' | 'teams_webhook'
+  webhook_url: string | null
+  email_addresses: string[] | null
+  is_active: boolean
+  events_enabled: Record<string, boolean>
+  created_at: string
+  updated_at: string
+}
+
+export interface NotificationConfigCreate {
+  channel: 'email' | 'slack_webhook' | 'teams_webhook'
+  webhook_url?: string
+  email_addresses?: string[]
+  events_enabled?: Record<string, boolean>
+}
+
+export interface NotificationConfigUpdate {
+  webhook_url?: string
+  email_addresses?: string[]
+  is_active?: boolean
+  events_enabled?: Record<string, boolean>
+}
+
+export interface AppNotification {
+  id: string
+  event: string
+  title: string
+  body: string | null
+  link: string | null
+  is_read: boolean
+  created_at: string
+}
+
+export interface UnreadCount {
+  count: number
+}
+
+export const NOTIFICATION_EVENTS = {
+  'session.escalated': 'Session Escalated',
+  'session.high_priority': 'High Priority Session',
+  'proposal.pending': 'New Flow Proposal',
+  'proposal.approved': 'Proposal Approved',
+  'knowledge_gap.detected': 'Knowledge Gap Detected',
+} as const
+
+export const CHANNEL_LABELS = {
+  email: 'Email',
+  slack_webhook: 'Slack',
+  teams_webhook: 'Microsoft Teams',
+} as const