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/entities/personality.entity.ts

@@ -1,15 +1,20 @@
-import type { Personality as PrismaPersonality, FormalityLevel } from "@prisma/client";
+import type { Personality as PrismaPersonality } from "@prisma/client";
 
+/**
+ * Personality entity representing an assistant configuration
+ */
 export class Personality implements PrismaPersonality {
   id!: string;
   workspaceId!: string;
-  name!: string;
+  name!: string; // unique identifier slug
+  displayName!: string; // human-readable name
   description!: string | null;
-  tone!: string;
-  formalityLevel!: FormalityLevel;
-  systemPromptTemplate!: string;
+  systemPrompt!: string;
+  temperature!: number | null; // null = use provider default
+  maxTokens!: number | null; // null = use provider default
+  llmProviderInstanceId!: string | null; // FK to LlmProviderInstance
   isDefault!: boolean;
-  isActive!: boolean;
+  isEnabled!: boolean;
   createdAt!: Date;
   updatedAt!: Date;
 }