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/mcp.rs

@@ -0,0 +1,97 @@
+//! MCP 服务器数据访问对象
+//!
+//! 提供 MCP 服务器的 CRUD 操作。
+
+use crate::app_config::{McpApps, McpServer};
+use crate::database::{lock_conn, Database};
+use crate::error::AppError;
+use indexmap::IndexMap;
+use rusqlite::params;
+
+impl Database {
+    /// 获取所有 MCP 服务器
+    pub fn get_all_mcp_servers(&self) -> Result<IndexMap<String, McpServer>, AppError> {
+        let conn = lock_conn!(self.conn);
+        let mut stmt = conn.prepare(
+            "SELECT id, name, server_config, description, homepage, docs, tags, enabled_claude, enabled_codex, enabled_gemini
+             FROM mcp_servers
+             ORDER BY name ASC, id ASC"
+        ).map_err(|e| AppError::Database(e.to_string()))?;
+
+        let server_iter = stmt
+            .query_map([], |row| {
+                let id: String = row.get(0)?;
+                let name: String = row.get(1)?;
+                let server_config_str: String = row.get(2)?;
+                let description: Option<String> = row.get(3)?;
+                let homepage: Option<String> = row.get(4)?;
+                let docs: Option<String> = row.get(5)?;
+                let tags_str: String = row.get(6)?;
+                let enabled_claude: bool = row.get(7)?;
+                let enabled_codex: bool = row.get(8)?;
+                let enabled_gemini: bool = row.get(9)?;
+
+                let server = serde_json::from_str(&server_config_str).unwrap_or_default();
+                let tags = serde_json::from_str(&tags_str).unwrap_or_default();
+
+                Ok((
+                    id.clone(),
+                    McpServer {
+                        id,
+                        name,
+                        server,
+                        apps: McpApps {
+                            claude: enabled_claude,
+                            codex: enabled_codex,
+                            gemini: enabled_gemini,
+                        },
+                        description,
+                        homepage,
+                        docs,
+                        tags,
+                    },
+                ))
+            })
+            .map_err(|e| AppError::Database(e.to_string()))?;
+
+        let mut servers = IndexMap::new();
+        for server_res in server_iter {
+            let (id, server) = server_res.map_err(|e| AppError::Database(e.to_string()))?;
+            servers.insert(id, server);
+        }
+        Ok(servers)
+    }
+
+    /// 保存 MCP 服务器
+    pub fn save_mcp_server(&self, server: &McpServer) -> Result<(), AppError> {
+        let conn = lock_conn!(self.conn);
+        conn.execute(
+            "INSERT OR REPLACE INTO mcp_servers (
+                id, name, server_config, description, homepage, docs, tags,
+                enabled_claude, enabled_codex, enabled_gemini
+            ) VALUES (?1, ?2, ?3, ?4, ?5, ?6, ?7, ?8, ?9, ?10)",
+            params![
+                server.id,
+                server.name,
+                serde_json::to_string(&server.server).unwrap(),
+                server.description,
+                server.homepage,
+                server.docs,
+                serde_json::to_string(&server.tags).unwrap(),
+                server.apps.claude,
+                server.apps.codex,
+                server.apps.gemini,
+            ],
+        )
+        .map_err(|e| AppError::Database(e.to_string()))?;
+        Ok(())
+    }
+
+    /// 删除 MCP 服务器
+    pub fn delete_mcp_server(&self, id: &str) -> Result<(), AppError> {
+        let conn = lock_conn!(self.conn);
+        conn.execute("DELETE FROM mcp_servers WHERE id = ?1", params![id])
+            .map_err(|e| AppError::Database(e.to_string()))?;
+        Ok(())
+    }
+}