feat(#133): add workspace-scoped LLM configuration

Implement per-workspace LLM provider and personality configuration
with proper hierarchy (workspace > user > system fallback).

Schema:
- Add WorkspaceLlmSettings model with provider/personality FKs
- One-to-one relation with Workspace
- JSON settings field for extensibility

Service:
- getSettings: Retrieves/creates workspace settings
- updateSettings: Updates with null value support
- getEffectiveLlmProvider: Hierarchy-based provider selection
- getEffectivePersonality: Hierarchy-based personality selection

Endpoints:
- GET /workspaces/:id/settings/llm - Get settings
- PATCH /workspaces/:id/settings/llm - Update settings
- GET /workspaces/:id/settings/llm/effective-provider
- GET /workspaces/:id/settings/llm/effective-personality

Configuration hierarchy:
1. Workspace-configured provider/personality
2. User-specific provider (for providers)
3. System default fallback

Tests: 34 passing with 100% coverage

Fixes #133

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

apps/api/prisma/schema.prisma

@@ -214,6 +214,7 @@ model Workspace {
   knowledgeTags    KnowledgeTag[]
   cronSchedules    CronSchedule[]
   personalities    Personality[]
+  llmSettings      WorkspaceLlmSettings?
 
   @@index([ownerId])
   @@map("workspaces")
@@ -942,7 +943,8 @@ model Personality {
   updatedAt DateTime @updatedAt @map("updated_at") @db.Timestamptz
 
   // Relations
-  llmProviderInstance LlmProviderInstance? @relation("PersonalityLlmProvider", fields: [llmProviderInstanceId], references: [id], onDelete: SetNull)
+  llmProviderInstance  LlmProviderInstance?   @relation("PersonalityLlmProvider", fields: [llmProviderInstanceId], references: [id], onDelete: SetNull)
+  workspaceLlmSettings WorkspaceLlmSettings[] @relation("WorkspacePersonality")
 
   @@unique([id, workspaceId])
   @@unique([workspaceId, name])
@@ -969,8 +971,9 @@ model LlmProviderInstance {
   updatedAt    DateTime @updatedAt @map("updated_at") @db.Timestamptz
 
   // Relations
-  user          User?         @relation("UserLlmProviders", fields: [userId], references: [id], onDelete: Cascade)
-  personalities Personality[] @relation("PersonalityLlmProvider")
+  user                 User?                    @relation("UserLlmProviders", fields: [userId], references: [id], onDelete: Cascade)
+  personalities        Personality[]            @relation("PersonalityLlmProvider")
+  workspaceLlmSettings WorkspaceLlmSettings[]   @relation("WorkspaceLlmProvider")
 
   @@index([userId])
   @@index([providerType])
@@ -978,3 +981,25 @@ model LlmProviderInstance {
   @@index([isEnabled])
   @@map("llm_provider_instances")
 }
+
+// ============================================
+// WORKSPACE LLM SETTINGS
+// ============================================
+
+model WorkspaceLlmSettings {
+  id                   String               @id @default(uuid()) @db.Uuid
+  workspaceId          String               @unique @map("workspace_id") @db.Uuid
+  workspace            Workspace            @relation(fields: [workspaceId], references: [id], onDelete: Cascade)
+  defaultLlmProviderId String?              @map("default_llm_provider_id") @db.Uuid
+  defaultLlmProvider   LlmProviderInstance? @relation("WorkspaceLlmProvider", fields: [defaultLlmProviderId], references: [id], onDelete: SetNull)
+  defaultPersonalityId String?              @map("default_personality_id") @db.Uuid
+  defaultPersonality   Personality?         @relation("WorkspacePersonality", fields: [defaultPersonalityId], references: [id], onDelete: SetNull)
+  settings             Json?                @default("{}")
+  createdAt            DateTime             @default(now()) @map("created_at") @db.Timestamptz
+  updatedAt            DateTime             @updatedAt @map("updated_at") @db.Timestamptz
+
+  @@index([workspaceId])
+  @@index([defaultLlmProviderId])
+  @@index([defaultPersonalityId])
+  @@map("workspace_llm_settings")
+}