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>

apps/api/prisma/migrations/20260131115600_add_llm_provider_instance/migration.sql

@@ -0,0 +1,29 @@
+-- CreateTable
+CREATE TABLE "llm_provider_instances" (
+    "id" UUID NOT NULL,
+    "provider_type" TEXT NOT NULL,
+    "display_name" TEXT NOT NULL,
+    "user_id" UUID,
+    "config" JSONB NOT NULL,
+    "is_default" BOOLEAN NOT NULL DEFAULT false,
+    "is_enabled" BOOLEAN NOT NULL DEFAULT true,
+    "created_at" TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updated_at" TIMESTAMPTZ NOT NULL,
+
+    CONSTRAINT "llm_provider_instances_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE INDEX "llm_provider_instances_user_id_idx" ON "llm_provider_instances"("user_id");
+
+-- CreateIndex
+CREATE INDEX "llm_provider_instances_provider_type_idx" ON "llm_provider_instances"("provider_type");
+
+-- CreateIndex
+CREATE INDEX "llm_provider_instances_is_default_idx" ON "llm_provider_instances"("is_default");
+
+-- CreateIndex
+CREATE INDEX "llm_provider_instances_is_enabled_idx" ON "llm_provider_instances"("is_enabled");
+
+-- AddForeignKey
+ALTER TABLE "llm_provider_instances" ADD CONSTRAINT "llm_provider_instances_user_id_fkey" FOREIGN KEY ("user_id") REFERENCES "users"("id") ON DELETE CASCADE ON UPDATE CASCADE;

apps/api/prisma/schema.prisma

@@ -168,6 +168,7 @@ model User {
   userLayouts             UserLayout[]
   userPreference          UserPreference?
   knowledgeEntryVersions  KnowledgeEntryVersion[]  @relation("EntryVersionAuthor")
+  llmProviders            LlmProviderInstance[]    @relation("UserLlmProviders")
 
   @@map("users")
 }
@@ -944,3 +945,28 @@ model Personality {
   @@index([workspaceId, isActive])
   @@map("personalities")
 }
+
+// ============================================
+// LLM PROVIDER MODULE
+// ============================================
+
+model LlmProviderInstance {
+  id           String  @id @default(uuid()) @db.Uuid
+  providerType String  @map("provider_type")  // "ollama" | "claude" | "openai"
+  displayName  String  @map("display_name")
+  userId       String? @map("user_id") @db.Uuid // 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") @db.Timestamptz
+  updatedAt    DateTime @updatedAt @map("updated_at") @db.Timestamptz
+
+  // Relations
+  user User? @relation("UserLlmProviders", fields: [userId], references: [id], onDelete: Cascade)
+
+  @@index([userId])
+  @@index([providerType])
+  @@index([isDefault])
+  @@index([isEnabled])
+  @@map("llm_provider_instances")
+}