refactor(database): split monolithic database.rs into modular structure

Split the 1788-line database.rs into a well-organized module directory:

- mod.rs (142 lines): Database struct and initialization
- schema.rs (341 lines): Table creation and schema migrations
- backup.rs (324 lines): SQL import/export and snapshot backup
- migration.rs (240 lines): JSON to SQLite data migration
- tests.rs (280 lines): Unit tests
- dao/providers.rs (254 lines): Provider CRUD operations
- dao/mcp.rs (97 lines): MCP server CRUD operations
- dao/prompts.rs (88 lines): Prompt CRUD operations
- dao/skills.rs (124 lines): Skills CRUD operations
- dao/settings.rs (65 lines): Settings key-value storage

Benefits:
- Max file size reduced from 1788 to 341 lines
- Clear separation of concerns (schema, backup, migration, DAOs)
- Easier to navigate and maintain
- All 107 tests passing with zero API changes

src-tauri/src/database/mod.rs

@@ -0,0 +1,142 @@
+//! 数据库模块 - SQLite 数据持久化
+//!
+//! 此模块提供应用的核心数据存储功能，包括：
+//! - 供应商配置管理
+//! - MCP 服务器配置
+//! - 提示词管理
+//! - Skills 管理
+//! - 通用设置存储
+//!
+//! ## 架构设计
+//!
+//! ```text
+//! database/
+//! ├── mod.rs        - Database 结构体 + 初始化
+//! ├── schema.rs     - 表结构定义 + Schema 迁移
+//! ├── backup.rs     - SQL 导入导出 + 快照备份
+//! ├── migration.rs  - JSON → SQLite 数据迁移
+//! └── dao/          - 数据访问对象
+//!     ├── providers.rs
+//!     ├── mcp.rs
+//!     ├── prompts.rs
+//!     ├── skills.rs
+//!     └── settings.rs
+//! ```
+
+mod backup;
+mod dao;
+mod migration;
+mod schema;
+
+#[cfg(test)]
+mod tests;
+
+use crate::config::get_app_config_dir;
+use crate::error::AppError;
+use rusqlite::Connection;
+use serde::Serialize;
+use std::sync::Mutex;
+
+// DAO 方法通过 impl Database 提供，无需额外导出
+
+/// 数据库备份保留数量
+const DB_BACKUP_RETAIN: usize = 10;
+
+/// 当前 Schema 版本号
+/// 每次修改表结构时递增，并在 schema.rs 中添加相应的迁移逻辑
+pub(crate) const SCHEMA_VERSION: i32 = 1;
+
+/// 安全地序列化 JSON，避免 unwrap panic
+pub(crate) fn to_json_string<T: Serialize>(value: &T) -> Result<String, AppError> {
+    serde_json::to_string(value)
+        .map_err(|e| AppError::Config(format!("JSON serialization failed: {e}")))
+}
+
+/// 安全地获取 Mutex 锁，避免 unwrap panic
+macro_rules! lock_conn {
+    ($mutex:expr) => {
+        $mutex
+            .lock()
+            .map_err(|e| AppError::Database(format!("Mutex lock failed: {}", e)))?
+    };
+}
+
+// 导出宏供子模块使用
+pub(crate) use lock_conn;
+
+/// 数据库连接封装
+///
+/// 使用 Mutex 包装 Connection 以支持在多线程环境（如 Tauri State）中共享。
+/// rusqlite::Connection 本身不是 Sync 的，因此需要这层包装。
+pub struct Database {
+    pub(crate) conn: Mutex<Connection>,
+}
+
+impl Database {
+    /// 初始化数据库连接并创建表
+    ///
+    /// 数据库文件位于 `~/.cc-switch/cc-switch.db`
+    pub fn init() -> Result<Self, AppError> {
+        let db_path = get_app_config_dir().join("cc-switch.db");
+
+        // 确保父目录存在
+        if let Some(parent) = db_path.parent() {
+            std::fs::create_dir_all(parent).map_err(|e| AppError::io(parent, e))?;
+        }
+
+        let conn = Connection::open(&db_path).map_err(|e| AppError::Database(e.to_string()))?;
+
+        // 启用外键约束
+        conn.execute("PRAGMA foreign_keys = ON;", [])
+            .map_err(|e| AppError::Database(e.to_string()))?;
+
+        let db = Self {
+            conn: Mutex::new(conn),
+        };
+        db.create_tables()?;
+        db.apply_schema_migrations()?;
+
+        Ok(db)
+    }
+
+    /// 创建内存数据库（用于测试）
+    pub fn memory() -> Result<Self, AppError> {
+        let conn = Connection::open_in_memory().map_err(|e| AppError::Database(e.to_string()))?;
+
+        // 启用外键约束
+        conn.execute("PRAGMA foreign_keys = ON;", [])
+            .map_err(|e| AppError::Database(e.to_string()))?;
+
+        let db = Self {
+            conn: Mutex::new(conn),
+        };
+        db.create_tables()?;
+
+        Ok(db)
+    }
+
+    /// 检查数据库是否为空（需要首次导入）
+    ///
+    /// 通过检查是否有任何 MCP 服务器、提示词、Skills 仓库或供应商来判断
+    pub fn is_empty_for_first_import(&self) -> Result<bool, AppError> {
+        let conn = lock_conn!(self.conn);
+
+        let mcp_count: i64 = conn
+            .query_row("SELECT COUNT(*) FROM mcp_servers", [], |row| row.get(0))
+            .map_err(|e| AppError::Database(e.to_string()))?;
+
+        let prompt_count: i64 = conn
+            .query_row("SELECT COUNT(*) FROM prompts", [], |row| row.get(0))
+            .map_err(|e| AppError::Database(e.to_string()))?;
+
+        let skill_repo_count: i64 = conn
+            .query_row("SELECT COUNT(*) FROM skill_repos", [], |row| row.get(0))
+            .map_err(|e| AppError::Database(e.to_string()))?;
+
+        let provider_count: i64 = conn
+            .query_row("SELECT COUNT(*) FROM providers", [], |row| row.get(0))
+            .map_err(|e| AppError::Database(e.to_string()))?;
+
+        Ok(mcp_count == 0 && prompt_count == 0 && skill_repo_count == 0 && provider_count == 0)
+    }
+}