feat: 新增 MySQL 数据库驱动支持，支持跨设备数据同步

backend/migrations/mysql/20260421000001_initial_schema.sql

@@ -0,0 +1,44 @@
+-- +goose Up
+-- MySQL 方言初始迁移：providers、models、usage_stats 完整表结构
+
+CREATE TABLE IF NOT EXISTS providers (
+    id VARCHAR(36) PRIMARY KEY,
+    name VARCHAR(255) NOT NULL,
+    api_key VARCHAR(255) NOT NULL,
+    base_url VARCHAR(255) NOT NULL,
+    protocol VARCHAR(50) DEFAULT 'openai',
+    enabled BOOLEAN DEFAULT TRUE,
+    created_at DATETIME(3),
+    updated_at DATETIME(3)
+);
+
+CREATE TABLE IF NOT EXISTS models (
+    id VARCHAR(36) PRIMARY KEY,
+    provider_id VARCHAR(36) NOT NULL,
+    model_name VARCHAR(255) NOT NULL,
+    enabled BOOLEAN DEFAULT TRUE,
+    created_at DATETIME(3),
+    FOREIGN KEY (provider_id) REFERENCES providers(id) ON DELETE CASCADE,
+    UNIQUE(provider_id, model_name)
+);
+
+CREATE TABLE IF NOT EXISTS usage_stats (
+    id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
+    provider_id VARCHAR(36) NOT NULL,
+    model_name VARCHAR(255) NOT NULL,
+    request_count INT DEFAULT 0,
+    date DATE NOT NULL,
+    UNIQUE(provider_id, model_name, date)
+);
+
+CREATE INDEX idx_models_provider_id ON models(provider_id);
+CREATE INDEX idx_models_model_name ON models(model_name);
+CREATE INDEX idx_usage_stats_provider_model_date ON usage_stats(provider_id, model_name, date);
+
+-- +goose Down
+DROP INDEX IF EXISTS idx_usage_stats_provider_model_date ON usage_stats;
+DROP INDEX IF EXISTS idx_models_model_name ON models;
+DROP INDEX IF EXISTS idx_models_provider_id ON models;
+DROP TABLE IF EXISTS usage_stats;
+DROP TABLE IF EXISTS models;
+DROP TABLE IF EXISTS providers;