feat(#273): Implement capability-based authorization for federation

Add CapabilityGuard infrastructure to enforce capability-based authorization
on federation endpoints. Implements fail-closed security model.

Security properties:
- Deny by default (no capability = deny)
- Only explicit true values grant access
- Connection must exist and be ACTIVE
- All denials logged for audit trail

Implementation:
- Created CapabilityGuard with fail-closed authorization logic
- Added @RequireCapability decorator for marking endpoints
- Added getConnectionById() to ConnectionService
- Added logCapabilityDenied() to AuditService
- 12 comprehensive tests covering all security scenarios

Quality gates:
- ✅ Tests: 12/12 passing
- ✅ Lint: 0 new errors (33 pre-existing)
- ✅ TypeScript: 0 new errors (8 pre-existing)

Refs #273

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/api/src/federation/guards/index.ts

@@ -0,0 +1,5 @@
+/**
+ * Federation Guards Exports
+ */
+
+export * from "./capability.guard";