refactor(mtproto): manage ad-tags per client only

The inbound-level ad-tag duplicated the per-client override for no
gain: the fork's global tag applied to every secret anyway, so one
value had two homes and they could drift. The inbound form field, the
settings key, and the global ad-tag in the generated config and in the
PUT /secrets body are gone; the tag is set on each client instead.
Existing inbound-level values are intentionally not migrated; a
leftover settings key is stripped on the next save.

internal/web/translation/zh-CN.json

@@ -545,8 +545,6 @@
         "mtprotoFakeTlsDomainHint": "生成新客户端密钥时使用的默认 FakeTLS 域名。每个客户端可使用各自的域名。",
         "mtgThrottleMaxConnections": "最大连接数",
         "mtgThrottleMaxConnectionsHint": "按公平分配限制所有用户的并发连接数。0 表示不限制。",
-        "mtgAdTag": "广告标签（赞助频道）",
-        "mtgAdTagHint": "可选的 32 位十六进制标签，从 Telegram 代理注册处获取。设置后，客户端将通过 Telegram 中间代理路由，赞助频道会显示在其聊天列表顶部。需要可访问的公网 IP —— 在下方设置，或让 mtg 自动检测。",
         "mtgAdTagInvalid": "广告标签必须为 32 位十六进制字符。",
         "mtgPublicIpv4": "公网 IPv4",
         "mtgPublicIpv6": "公网 IPv6",
@@ -919,7 +917,7 @@
       "mtprotoSecret": "MTProto 密钥",
       "mtprotoSecretHint": "该客户端的 FakeTLS 密钥。重新生成即可更换。",
       "mtprotoAdTag": "广告标签（赞助频道）",
-      "mtprotoAdTagHint": "可选的 32 位十六进制标签，仅对该客户端生效，覆盖共享广告标签。留空则使用共享标签。",
+      "mtprotoAdTagHint": "可选的 32 位十六进制标签，从 Telegram 代理注册处获取。设置后，该客户端将通过 Telegram 中间代理路由，赞助频道会显示在其聊天列表顶部。",
       "reverseTag": "反向标签",
       "reverseTagPlaceholder": "可选 Reverse tag",
       "telegramId": "Telegram 用户 ID",