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.

frontend/src/schemas/protocols/inbound/mtproto.ts

@@ -57,15 +57,8 @@ export const MtprotoInboundSettingsSchema = z.object({
   routeThroughXray: z.boolean().optional(),
   outboundTag: z.string().optional(),
   routeXrayPort: z.number().int().min(0).max(65535).optional(),
-  // A 32-hex Telegram advertising tag: when set, mtg routes clients through
-  // Telegram middle proxies so a sponsored channel appears in their chat list.
-  // publicIpv4/publicIpv6 pin this server's reachable address the middle proxy
-  // needs; leave them blank to let mtg auto-detect it.
-  adTag: z
-    .string()
-    .regex(/^[0-9a-fA-F]{32}$/, 'pages.inbounds.form.mtgAdTagInvalid')
-    .or(z.literal(''))
-    .optional(),
+  // publicIpv4/publicIpv6 pin this server's reachable address the Telegram
+  // middle proxy needs when clients carry ad-tags; blank = mtg auto-detects.
   publicIpv4: z.string().optional(),
   publicIpv6: z.string().optional(),
 });