feat(#139): build Gate Rejection Response Handler

Implement rejection handling for tasks that fail quality gates after
all continuation attempts are exhausted.

Schema:
- Add TaskRejection model for tracking rejections
- Store failures, attempts, escalation state

Service:
- handleRejection: Main entry point for rejection handling
- logRejection: Database logging
- determineEscalation: Rule-based escalation determination
- executeEscalation: Execute escalation actions
- sendNotification: Notification dispatch
- markForManualReview: Flag tasks for human review
- getRejectionHistory: Query rejection history
- generateRejectionReport: Markdown report generation

Escalation rules:
- max-attempts: Trigger after 3+ attempts
- time-exceeded: Trigger after 2+ hours
- critical-failure: Trigger on security/critical issues

Actions: notify, block, reassign, cancel

Tests: 16 passing with 80% statement coverage

Fixes #139

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

apps/api/src/rejection-handler/index.ts

@@ -0,0 +1,3 @@
+export * from "./rejection-handler.module";
+export * from "./rejection-handler.service";
+export * from "./interfaces";