feat(#128): add LlmProviderInstance Prisma schema

Added database schema for LLM provider instance configuration to support
multi-provider architecture.

Schema design:
- LlmProviderInstance model with UUID primary key
- Fields: providerType, displayName, userId, config, isDefault, isEnabled
- JSON config field for flexible provider-specific settings
- Nullable userId: NULL = system-level, UUID = user-level
- Foreign key to User with CASCADE delete
- Added llmProviders relation to User model

Indexes:
- user_id: Fast user lookup
- provider_type: Filter by provider
- is_default: Quick default lookup
- is_enabled: Enabled/disabled filtering

Migration: 20260131115600_add_llm_provider_instance
- PostgreSQL table creation with proper types
- Foreign key constraint
- Performance indexes

Prisma client regenerated successfully.
Database migration requires manual deployment when DB is available.

Fixes #128

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

docs/scratchpads/128-llm-provider-schema.md

@@ -0,0 +1,120 @@
+# Issue #128: Add LlmProviderInstance Prisma Schema
+
+## Objective
+
+Create database schema for LLM provider instance configuration to support multi-provider LLM backend.
+
+## Approach
+
+### Schema Design
+
+```prisma
+model LlmProviderInstance {
+  id            String   @id @default(uuid())
+  providerType  String   @map("provider_type")  // "ollama" | "claude" | "openai"
+  displayName   String   @map("display_name")
+  userId        String?  @map("user_id")        // NULL = system-level, UUID = user-level
+  config        Json                             // Provider-specific configuration
+  isDefault     Boolean  @default(false) @map("is_default")
+  isEnabled     Boolean  @default(true) @map("is_enabled")
+  createdAt     DateTime @default(now()) @map("created_at")
+  updatedAt     DateTime @updatedAt @map("updated_at")
+
+  user          User?    @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@index([userId])
+  @@index([providerType])
+  @@index([isDefault])
+  @@map("llm_provider_instances")
+}
+```
+
+### Configuration Patterns
+
+**System-level instances** (user_id = NULL):
+
+- Available to all users
+- Managed by admins only
+- Example: Shared Ollama server
+
+**User-level instances** (user_id = UUID):
+
+- Private to specific user
+- User manages their own API keys
+- Example: Personal OpenAI API key
+
+### Config Field Structure
+
+```json
+{
+  "endpoint": "http://localhost:11434",
+  "timeout": 30000,
+  "apiKey": "sk-...", // encrypted in production
+  "organization": "org-...",
+  "customHeaders": {}
+}
+```
+
+## Progress
+
+- [x] Read current Prisma schema
+- [x] Add LlmProviderInstance model
+- [x] Add relation to User model
+- [x] Add indexes for performance
+- [x] Create migration (20260131115600_add_llm_provider_instance)
+- [x] Update Prisma client types (prisma generate)
+- [ ] Run migration on dev database (manual step, requires DB connection)
+- [ ] Verify migration succeeded (manual step, requires DB connection)
+
+## Testing
+
+- Verify model is accessible via Prisma client
+- Verify indexes exist in database
+- Verify foreign key constraints work
+- Test both system and user-level instances
+
+## Notes
+
+- Using snake_case for database columns (project convention)
+- JSON field for flexible provider configs
+- Cascade delete when user is deleted
+- `isDefault` allows marking one provider as default per user
+
+## Implementation Details
+
+**Model Added:**
+
+- `LlmProviderInstance` with all required fields
+- Foreign key relation to `User` model (nullable for system-level instances)
+- Added `llmProviders` relation array to User model
+
+**Indexes Created:**
+
+- `user_id` - Fast lookup by user
+- `provider_type` - Fast filtering by provider
+- `is_default` - Quick default provider lookup
+- `is_enabled` - Filter enabled/disabled providers
+
+**Migration File:** `20260131115600_add_llm_provider_instance`
+
+- Creates table with proper PostgreSQL types
+- Sets up foreign key constraint with CASCADE delete
+- Creates all performance indexes
+
+## Acceptance Criteria Status
+
+- ✅ Migration created successfully
+- ✅ Model accessible via Prisma client (prisma generate succeeded)
+- ✅ Indexes defined correctly
+- ⏸️ Migration not run (requires database connection)
+- ⏸️ No data loss risk (new table, no existing data)
+
+## Next Steps
+
+When database is available:
+
+```bash
+pnpm --filter @mosaic/api prisma migrate deploy
+# OR for development:
+pnpm --filter @mosaic/api prisma migrate dev
+```