feat: 实现分层架构，包含 domain、service、repository 和 pkg 层

- 新增 domain 层：model、provider、route、stats 实体
- 新增 service 层：models、providers、routing、stats 业务逻辑
- 新增 repository 层：models、providers、stats 数据访问
- 新增 pkg 工具包：errors、logger、validator
- 新增中间件：CORS、logging、recovery、request ID
- 新增数据库迁移：初始 schema 和索引
- 新增单元测试和集成测试
- 新增规范文档：config-management、database-migration、error-handling、layered-architecture、middleware-system、request-validation、structured-logging、test-coverage
- 移除 config 子包和 model_router（已迁移至分层架构）

backend/migrations/001_initial_schema.sql

@@ -0,0 +1,33 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS providers (
+    id TEXT PRIMARY KEY,
+    name TEXT NOT NULL,
+    api_key TEXT NOT NULL,
+    base_url TEXT NOT NULL,
+    enabled INTEGER DEFAULT 1,
+    created_at DATETIME,
+    updated_at DATETIME
+);
+
+CREATE TABLE IF NOT EXISTS models (
+    id TEXT PRIMARY KEY,
+    provider_id TEXT NOT NULL,
+    model_name TEXT NOT NULL,
+    enabled INTEGER DEFAULT 1,
+    created_at DATETIME,
+    FOREIGN KEY (provider_id) REFERENCES providers(id) ON DELETE CASCADE
+);
+
+CREATE TABLE IF NOT EXISTS usage_stats (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    provider_id TEXT NOT NULL,
+    model_name TEXT NOT NULL,
+    request_count INTEGER DEFAULT 0,
+    date DATE NOT NULL,
+    UNIQUE(provider_id, model_name, date)
+);
+
+-- +goose Down
+DROP TABLE IF EXISTS usage_stats;
+DROP TABLE IF EXISTS models;
+DROP TABLE IF EXISTS providers;