feat(#85): implement CONNECT/DISCONNECT protocol

Implemented connection handshake protocol for federation building on
the Instance Identity Model from issue #84.

**Services:**
- SignatureService: Message signing/verification with RSA-SHA256
- ConnectionService: Federation connection management

**API 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
- GET /api/v1/federation/connections
- GET /api/v1/federation/connections/:id
- POST /api/v1/federation/incoming/connect

**Tests:** 70 tests pass (18 Signature + 20 Connection + 13 Controller + 19 existing)
**Coverage:** 100% on new code
**TDD Approach:** Tests written before implementation

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

apps/api/package.json

@@ -26,6 +26,7 @@
   "dependencies": {
     "@anthropic-ai/sdk": "^0.72.1",
     "@mosaic/shared": "workspace:*",
+    "@nestjs/axios": "^4.0.1",
     "@nestjs/bullmq": "^11.0.4",
     "@nestjs/common": "^11.1.12",
     "@nestjs/config": "^4.0.2",
@@ -47,6 +48,7 @@
     "@types/multer": "^2.0.0",
     "adm-zip": "^0.5.16",
     "archiver": "^7.0.1",
+    "axios": "^1.13.4",
     "better-auth": "^1.4.17",
     "bullmq": "^5.67.2",
     "class-transformer": "^0.5.1",