feat(#87): implement cross-instance identity linking for federation

Implements FED-004: Cross-Instance Identity Linking, building on the
foundation from FED-001, FED-002, and FED-003.

New Services:
- IdentityLinkingService: Handles identity verification and mapping
  with signature validation and OIDC token verification
- IdentityResolutionService: Resolves identities between local and
  remote instances with support for bulk operations

New API Endpoints (IdentityLinkingController):
- POST /api/v1/federation/identity/verify - Verify remote identity
- POST /api/v1/federation/identity/resolve - Resolve remote to local user
- POST /api/v1/federation/identity/bulk-resolve - Bulk resolution
- GET /api/v1/federation/identity/me - Get current user's identities
- POST /api/v1/federation/identity/link - Create identity mapping
- PATCH /api/v1/federation/identity/:id - Update mapping
- DELETE /api/v1/federation/identity/:id - Revoke mapping
- GET /api/v1/federation/identity/:id/validate - Validate mapping

Security Features:
- Signature verification using remote instance public keys
- OIDC token validation before creating mappings
- Timestamp validation to prevent replay attacks
- Workspace isolation via authentication guards
- Comprehensive audit logging for all identity operations

Enhancements:
- Added SignatureService.verifyMessage() for remote signature verification
- Added FederationService.getConnectionByRemoteInstanceId()
- Extended FederationAuditService with identity logging methods
- Created comprehensive DTOs with class-validator decorators

Testing:
- 38 new tests (19 service + 7 resolution + 12 controller)
- All 132 federation tests passing
- TypeScript compilation passing with no errors
- High test coverage achieved (>85% requirement exceeded)

Technical Details:
- Leverages existing FederatedIdentity model from FED-003
- Uses RSA SHA-256 signatures for cryptographic verification
- Supports one identity mapping per remote instance per user
- Resolution service optimized for read-heavy operations
- Built following TDD principles (Red-Green-Refactor)

Closes #87

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

apps/api/src/federation/signature.service.ts

@@ -116,6 +116,40 @@ export class SignatureService {
     return this.sign(message, identity.privateKey);
   }
 
+  /**
+   * Verify a message signature using a remote instance's public key
+   * Fetches the public key from the connection record
+   */
+  async verifyMessage(
+    message: SignableMessage,
+    signature: string,
+    remoteInstanceId: string
+  ): Promise<SignatureValidationResult> {
+    try {
+      // Fetch remote instance public key from connection record
+      // For now, we'll fetch from any connection with this instance
+      // In production, this should be cached or fetched from instance identity endpoint
+      const connection =
+        await this.federationService.getConnectionByRemoteInstanceId(remoteInstanceId);
+
+      if (!connection) {
+        return {
+          valid: false,
+          error: "Remote instance not connected",
+        };
+      }
+
+      // Verify signature using remote public key
+      return this.verify(message, signature, connection.remotePublicKey);
+    } catch (error) {
+      this.logger.error("Failed to verify message", error);
+      return {
+        valid: false,
+        error: error instanceof Error ? error.message : "Verification failed",
+      };
+    }
+  }
+
   /**
    * Verify a connection request signature
    */