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/prisma/schema.prisma

@@ -921,28 +921,35 @@ model Personality {
   workspace   Workspace @relation(fields: [workspaceId], references: [id], onDelete: Cascade)
 
   // Identity
-  name        String
+  name        String   // unique identifier slug
+  displayName String   @map("display_name")
   description String?  @db.Text
 
-  // Personality traits
-  tone           String
-  formalityLevel FormalityLevel @map("formality_level")
+  // System prompt
+  systemPrompt String @map("system_prompt") @db.Text
 
-  // System prompt template
-  systemPromptTemplate String @map("system_prompt_template") @db.Text
+  // LLM configuration
+  temperature            Float?  // null = use provider default
+  maxTokens              Int?    @map("max_tokens") // null = use provider default
+  llmProviderInstanceId  String? @map("llm_provider_instance_id") @db.Uuid
 
   // Status
   isDefault Boolean @default(false) @map("is_default")
-  isActive  Boolean @default(true) @map("is_active")
+  isEnabled Boolean @default(true) @map("is_enabled")
 
   // Audit
   createdAt DateTime @default(now()) @map("created_at") @db.Timestamptz
   updatedAt DateTime @updatedAt @map("updated_at") @db.Timestamptz
 
+  // Relations
+  llmProviderInstance LlmProviderInstance? @relation("PersonalityLlmProvider", fields: [llmProviderInstanceId], references: [id], onDelete: SetNull)
+
   @@unique([id, workspaceId])
+  @@unique([workspaceId, name])
   @@index([workspaceId])
   @@index([workspaceId, isDefault])
-  @@index([workspaceId, isActive])
+  @@index([workspaceId, isEnabled])
+  @@index([llmProviderInstanceId])
   @@map("personalities")
 }
 
@@ -962,7 +969,8 @@ model LlmProviderInstance {
   updatedAt    DateTime @updatedAt @map("updated_at") @db.Timestamptz
 
   // Relations
-  user User? @relation("UserLlmProviders", fields: [userId], references: [id], onDelete: Cascade)
+  user          User?         @relation("UserLlmProviders", fields: [userId], references: [id], onDelete: Cascade)
+  personalities Personality[] @relation("PersonalityLlmProvider")
 
   @@index([userId])
   @@index([providerType])