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/dao/settings.rs

@@ -0,0 +1,65 @@
+//! 通用设置数据访问对象
+//!
+//! 提供键值对形式的通用设置存储。
+
+use crate::database::{lock_conn, Database};
+use crate::error::AppError;
+use rusqlite::params;
+
+impl Database {
+    /// 获取设置值
+    pub fn get_setting(&self, key: &str) -> Result<Option<String>, AppError> {
+        let conn = lock_conn!(self.conn);
+        let mut stmt = conn
+            .prepare("SELECT value FROM settings WHERE key = ?1")
+            .map_err(|e| AppError::Database(e.to_string()))?;
+
+        let mut rows = stmt
+            .query(params![key])
+            .map_err(|e| AppError::Database(e.to_string()))?;
+
+        if let Some(row) = rows.next().map_err(|e| AppError::Database(e.to_string()))? {
+            Ok(Some(
+                row.get(0).map_err(|e| AppError::Database(e.to_string()))?,
+            ))
+        } else {
+            Ok(None)
+        }
+    }
+
+    /// 设置值
+    pub fn set_setting(&self, key: &str, value: &str) -> Result<(), AppError> {
+        let conn = lock_conn!(self.conn);
+        conn.execute(
+            "INSERT OR REPLACE INTO settings (key, value) VALUES (?1, ?2)",
+            params![key, value],
+        )
+        .map_err(|e| AppError::Database(e.to_string()))?;
+        Ok(())
+    }
+
+    // --- Config Snippets 辅助方法 ---
+
+    /// 获取通用配置片段
+    pub fn get_config_snippet(&self, app_type: &str) -> Result<Option<String>, AppError> {
+        self.get_setting(&format!("common_config_{app_type}"))
+    }
+
+    /// 设置通用配置片段
+    pub fn set_config_snippet(
+        &self,
+        app_type: &str,
+        snippet: Option<String>,
+    ) -> Result<(), AppError> {
+        let key = format!("common_config_{app_type}");
+        if let Some(value) = snippet {
+            self.set_setting(&key, &value)
+        } else {
+            // 如果为 None 则删除
+            let conn = lock_conn!(self.conn);
+            conn.execute("DELETE FROM settings WHERE key = ?1", params![key])
+                .map_err(|e| AppError::Database(e.to_string()))?;
+            Ok(())
+        }
+    }
+}