feat(#130): add Personality Prisma schema and backend

Implement Personality system backend with database schema, service,
controller, and comprehensive tests. Personalities define assistant
behavior with system prompts and LLM configuration.

Changes:
- Update Personality model in schema.prisma with LLM provider relation
- Create PersonalitiesService with CRUD and default management
- Create PersonalitiesController with REST endpoints
- Add DTOs with validation (create/update)
- Add entity for type safety
- Remove unused PromptFormatterService
- Achieve 26 tests with full coverage

Endpoints:
- GET /personality - List all
- GET /personality/default - Get default
- GET /personality/by-name/:name - Get by name
- GET /personality/:id - Get one
- POST /personality - Create
- PATCH /personality/:id - Update
- DELETE /personality/:id - Delete
- POST /personality/:id/set-default - Set default

Fixes #130

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

apps/api/src/personalities/personalities.module.ts

@@ -3,12 +3,11 @@ import { PrismaModule } from "../prisma/prisma.module";
 import { AuthModule } from "../auth/auth.module";
 import { PersonalitiesService } from "./personalities.service";
 import { PersonalitiesController } from "./personalities.controller";
-import { PromptFormatterService } from "./services/prompt-formatter.service";
 
 @Module({
   imports: [PrismaModule, AuthModule],
   controllers: [PersonalitiesController],
-  providers: [PersonalitiesService, PromptFormatterService],
-  exports: [PersonalitiesService, PromptFormatterService],
+  providers: [PersonalitiesService],
+  exports: [PersonalitiesService],
 })
 export class PersonalitiesModule {}