feat: gateway CRUD routes for conversations, projects, missions, tasks (P1-005, P1-006)

- BrainModule provides Brain instance to all gateway controllers
- ConversationsController: CRUD + message listing/creation
- ProjectsController: CRUD with owner assignment
- MissionsController: CRUD with project association
- TasksController: CRUD with project/mission/status filtering
- All routes guarded by AuthGuard (BetterAuth session validation)
- DTOs at module boundaries per project convention

Closes #14, Closes #15

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/gateway/src/conversations/conversations.dto.ts

@@ -0,0 +1,15 @@
+export interface CreateConversationDto {
+  title?: string;
+  projectId?: string;
+}
+
+export interface UpdateConversationDto {
+  title?: string;
+  projectId?: string | null;
+}
+
+export interface SendMessageDto {
+  role: 'user' | 'assistant' | 'system';
+  content: string;
+  metadata?: Record<string, unknown>;
+}