fix(#278): Implement CSRF protection using double-submit cookie pattern

Implemented comprehensive CSRF protection for all state-changing endpoints
(POST, PATCH, DELETE) using the double-submit cookie pattern.

Security Implementation:
- Created CsrfGuard using double-submit cookie validation
- Token set in httpOnly cookie and validated against X-CSRF-Token header
- Applied guard to FederationController (vulnerable endpoints)
- Safe HTTP methods (GET, HEAD, OPTIONS) automatically exempted
- Signature-based endpoints (@SkipCsrf decorator) exempted

Components Added:
- CsrfGuard: Validates cookie and header token match
- CsrfController: GET /api/v1/csrf/token endpoint for token generation
- @SkipCsrf(): Decorator to exempt endpoints with alternative auth
- Comprehensive tests (20 tests, all passing)

Protected Endpoints:
- POST /api/v1/federation/connections/initiate
- POST /api/v1/federation/connections/:id/accept
- POST /api/v1/federation/connections/:id/reject
- POST /api/v1/federation/connections/:id/disconnect
- POST /api/v1/federation/instance/regenerate-keys

Exempted Endpoints:
- POST /api/v1/federation/incoming/connect (signature-verified)
- GET requests (safe methods)

Security Features:
- httpOnly cookies prevent XSS attacks
- SameSite=strict prevents subdomain attacks
- Cryptographically secure random tokens (32 bytes)
- 24-hour token expiry
- Structured logging for security events

Testing:
- 14 guard tests covering all scenarios
- 6 controller tests for token generation
- Quality gates: lint, typecheck, build all passing

Note: Frontend integration required to use tokens. Clients must:
1. GET /api/v1/csrf/token to receive token
2. Include token in X-CSRF-Token header for state-changing requests

Fixes #278

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

apps/api/src/app.module.ts

@@ -5,6 +5,7 @@ import { BullModule } from "@nestjs/bullmq";
 import { ThrottlerValkeyStorageService, ThrottlerApiKeyGuard } from "./common/throttler";
 import { AppController } from "./app.controller";
 import { AppService } from "./app.service";
+import { CsrfController } from "./common/controllers/csrf.controller";
 import { PrismaModule } from "./prisma/prisma.module";
 import { DatabaseModule } from "./database/database.module";
 import { AuthModule } from "./auth/auth.module";
@@ -87,7 +88,7 @@ import { FederationModule } from "./federation/federation.module";
     CoordinatorIntegrationModule,
     FederationModule,
   ],
-  controllers: [AppController],
+  controllers: [AppController, CsrfController],
   providers: [
     AppService,
     {