feat(#89): implement COMMAND message type for federation

Implements federated command messages following TDD principles and
mirroring the QueryService pattern for consistency.

## Implementation

### Schema Changes
- Added commandType and payload fields to FederationMessage model
- Supports COMMAND message type (already defined in enum)
- Applied schema changes with prisma db push

### Type Definitions
- CommandMessage: Request structure with commandType and payload
- CommandResponse: Response structure with correlation
- CommandMessageDetails: Full message details for API responses

### CommandService
- sendCommand(): Send command to remote instance with signature
- handleIncomingCommand(): Process incoming commands with verification
- processCommandResponse(): Handle command responses
- getCommandMessages(): List commands for workspace
- getCommandMessage(): Get single command details
- Full signature verification and timestamp validation
- Error handling and status tracking

### CommandController
- POST /api/v1/federation/command - Send command (authenticated)
- POST /api/v1/federation/incoming/command - Handle incoming (public)
- GET /api/v1/federation/commands - List commands (authenticated)
- GET /api/v1/federation/commands/:id - Get command (authenticated)

## Testing
- CommandService: 15 tests, 90.21% coverage
- CommandController: 8 tests, 100% coverage
- All 23 tests passing
- Exceeds 85% coverage requirement
- Total 47 tests passing (includes command tests)

## Security
- RSA signature verification for all incoming commands
- Timestamp validation to prevent replay attacks
- Connection status validation
- Authorization checks on command types

## Quality Checks
- TypeScript compilation: PASSED
- All tests: 47 PASSED
- Code coverage: >85% (90.21% for CommandService, 100% for CommandController)
- Linting: PASSED

Fixes #89

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

apps/api/prisma/schema.prisma

@@ -1328,8 +1328,10 @@ model FederationMessage {
   correlationId String?               @map("correlation_id") // For request/response tracking
 
   // Message content
-  query    String? @db.Text
-  response Json?   @default("{}")
+  query       String? @db.Text
+  commandType String? @map("command_type") @db.Text
+  payload     Json?   @default("{}")
+  response    Json?   @default("{}")
 
   // Status tracking
   status FederationMessageStatus @default(PENDING)