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/src/workspace-settings/workspace-settings.module.ts

@@ -0,0 +1,12 @@
+import { Module } from "@nestjs/common";
+import { WorkspaceSettingsController } from "./workspace-settings.controller";
+import { WorkspaceSettingsService } from "./workspace-settings.service";
+import { PrismaModule } from "../prisma/prisma.module";
+
+@Module({
+  imports: [PrismaModule],
+  controllers: [WorkspaceSettingsController],
+  providers: [WorkspaceSettingsService],
+  exports: [WorkspaceSettingsService],
+})
+export class WorkspaceSettingsModule {}