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/knowledge_flywheel.py

@@ -20,6 +20,7 @@ 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.models.flow_proposal import FlowProposal
@@ -295,6 +296,7 @@ async def _auto_reinforce(session: AISession, db: AsyncSession) -> None:
         target_flow_id=session.matched_flow_id,
     )
     db.add(proposal)
+    # auto_reinforced proposals don't need review — no notification
     logger.info("Auto-reinforced flow %s from session %s", session.matched_flow_id, session.id)
 
 
@@ -355,6 +357,12 @@ async def _propose_new_flow(session: AISession, db: AsyncSession) -> None:
         status="pending",
     )
     db.add(proposal)
+    await notify("proposal.pending", proposal.account_id, {
+        "title": proposal.title,
+        "proposal_type": proposal.proposal_type,
+        "problem_domain": proposal.problem_domain or "General",
+        "link": "/review-queue",
+    }, db)
     logger.info("Created new_flow proposal for session %s: %s", session.id, title)
 
 
@@ -431,6 +439,12 @@ async def _propose_enhancement(session: AISession, db: AsyncSession) -> None:
         status="pending",
     )
     db.add(proposal)
+    await notify("proposal.pending", proposal.account_id, {
+        "title": proposal.title,
+        "proposal_type": proposal.proposal_type,
+        "problem_domain": proposal.problem_domain or "General",
+        "link": "/review-queue",
+    }, db)
     logger.info(
         "Created enhancement proposal for flow %s from session %s: %s",
         session.matched_flow_id, session.id, title,