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/schemas/notification.py

@@ -0,0 +1,85 @@
+"""Pydantic schemas for notification system."""
+from datetime import datetime
+from uuid import UUID
+
+from pydantic import BaseModel, Field, field_validator
+
+
+VALID_EVENTS = {
+    "session.escalated",
+    "session.high_priority",
+    "proposal.pending",
+    "proposal.approved",
+    "knowledge_gap.detected",
+}
+
+
+class NotificationConfigCreate(BaseModel):
+    channel: str = Field(..., pattern="^(email|slack_webhook|teams_webhook)$")
+    webhook_url: str | None = None
+    email_addresses: list[str] | None = None
+    events_enabled: dict[str, bool] = Field(
+        default_factory=lambda: {e: True for e in VALID_EVENTS}
+    )
+
+    @field_validator("events_enabled")
+    @classmethod
+    def validate_event_keys(cls, v: dict[str, bool]) -> dict[str, bool]:
+        invalid = set(v) - VALID_EVENTS
+        if invalid:
+            raise ValueError(f"Unknown event keys: {invalid}")
+        return v
+
+
+class NotificationConfigUpdate(BaseModel):
+    webhook_url: str | None = None
+    email_addresses: list[str] | None = None
+    is_active: bool | None = None
+    events_enabled: dict[str, bool] | None = None
+
+    @field_validator("events_enabled")
+    @classmethod
+    def validate_event_keys(cls, v: dict[str, bool] | None) -> dict[str, bool] | None:
+        if v is not None:
+            invalid = set(v) - VALID_EVENTS
+            if invalid:
+                raise ValueError(f"Unknown event keys: {invalid}")
+        return v
+
+
+class NotificationConfigResponse(BaseModel):
+    id: UUID
+    channel: str
+    webhook_url: str | None
+    email_addresses: list[str] | None
+    is_active: bool
+    events_enabled: dict[str, bool]
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class NotificationResponse(BaseModel):
+    id: UUID
+    event: str
+    title: str
+    body: str | None
+    link: str | None
+    is_read: bool
+    created_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class UnreadCountResponse(BaseModel):
+    count: int
+
+
+class NotificationTestRequest(BaseModel):
+    config_id: UUID
+
+
+class NotificationTestResponse(BaseModel):
+    success: bool
+    message: str