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/src/federation/dto/connection.dto.ts

@@ -0,0 +1,64 @@
+/**
+ * Connection DTOs
+ *
+ * Data Transfer Objects for federation connection API.
+ */
+
+import { IsString, IsUrl, IsOptional, IsObject } from "class-validator";
+
+/**
+ * DTO for initiating a connection
+ */
+export class InitiateConnectionDto {
+  @IsUrl()
+  remoteUrl!: string;
+}
+
+/**
+ * DTO for accepting a connection
+ */
+export class AcceptConnectionDto {
+  @IsOptional()
+  @IsObject()
+  metadata?: Record<string, unknown>;
+}
+
+/**
+ * DTO for rejecting a connection
+ */
+export class RejectConnectionDto {
+  @IsString()
+  reason!: string;
+}
+
+/**
+ * DTO for disconnecting a connection
+ */
+export class DisconnectConnectionDto {
+  @IsOptional()
+  @IsString()
+  reason?: string;
+}
+
+/**
+ * DTO for incoming connection request (from remote instance)
+ */
+export class IncomingConnectionRequestDto {
+  @IsString()
+  instanceId!: string;
+
+  @IsUrl()
+  instanceUrl!: string;
+
+  @IsString()
+  publicKey!: string;
+
+  @IsObject()
+  capabilities!: Record<string, unknown>;
+
+  @IsString()
+  timestamp!: number;
+
+  @IsString()
+  signature!: string;
+}