feat(proxy): add proxy management endpoint and proxy creation API

- Implement new endpoint for creating frpc proxy configurations
- Add DBQueryFrpcInstanceByID function to fetch instances by ID
- Move proxy generation logic to separate function in frpcProxyAct.go
- Update API documentation with new proxy creation endpoint

docs/api.md

@@ -635,6 +635,92 @@ Modify system-level settings such as instance name, boot-at-start configuration,
 
 ---
 
+## Create Proxy
+
+**Endpoint:** `/frpcAct/proxyMgr/create`  
+**Method:** POST  
+**Content-Type:** application/json  
+**Auth Required:** Yes (token)  
+**Permission Level:** Admin
+
+Create a new proxy configuration for an existing frpc instance. The proxy configuration will be appended to the instance's config file.
+
+**Request Headers:**
+```
+X-Token: your_token
+X-Timestamp: 1704067200000
+```
+
+**Request Body:**
+```json
+{
+  "instanceID": "1",
+  "proxyInfo": {
+    "name": "ssh_proxy",
+    "type": "tcp",
+    "localIP": "127.0.0.1",
+    "localPort": "22",
+    "remotePort": "6000"
+  }
+}
+```
+
+| Header | Type | Required | Description |
+|--------|------|----------|-------------|
+| X-Token | string | Yes | Authentication token |
+| X-Timestamp | int64 | Yes | Client timestamp in milliseconds |
+
+| Field | Type | Required | Description |
+|-------|------|----------|-------------|
+| instanceID | string | Yes | Instance ID (the ID of the frpc instance) |
+| proxyInfo.name | string | Yes | Proxy name |
+| proxyInfo.type | string | Yes | Proxy type (e.g., tcp, udp, http, https) |
+| proxyInfo.localIP | string | Yes | Local IP address to forward to |
+| proxyInfo.localPort | string | Yes | Local port to forward from |
+| proxyInfo.remotePort | string | Yes | Remote port on frps to expose |
+
+**Response:**
+```json
+{
+  "success": true,
+  "message": "Proxy created successfully",
+  "data": {
+    "instanceName": "my_frpc",
+    "instanceID": 1,
+    "configPath": "./configs/superfrpc_user_my_frpc.toml",
+    "proxyName": "ssh_proxy"
+  }
+}
+```
+
+| Field | Type | Description |
+|-------|------|-------------|
+| instanceName | string | Name of the frpc instance |
+| instanceID | int | Instance ID |
+| configPath | string | Path to the configuration file |
+| proxyName | string | Name of the created proxy |
+
+**Config File Format:**
+
+The proxy configuration will be appended to the instance's config file in the following format:
+
+```toml
+[[proxies]]
+name = ssh_proxy
+type = tcp
+local_ip = 127.0.0.1
+local_port = 22
+remote_port = 6000
+```
+
+> **Note:** 
+> - The proxy configuration is appended to the existing config file
+> - The instance must already exist before creating a proxy
+> - This endpoint does not modify the database, only the config file
+> - The frpc service needs to be restarted for changes to take effect
+
+---
+
 ## List frpc Instances
 
 **Endpoint:** `/frpcAct/instanceMgr/list`