feat(mtproto): per-client ad-tags, management-API auth, and record secret sync

Catch the panel up to the mtg-multi README (v1.14.0):

- Each client can now carry its own 32-hex advertising tag overriding the
  inbound-level one. The tag lives on the client (settings JSON is the
  source of truth, clients.ad_tag is the UI projection), is rendered into
  the fork's [secret-ad-tags] section for active secrets only (mtg rejects
  a config whose override names an unknown secret), is pushed per entry
  through PUT /secrets, and is part of the reload fingerprint so a tag
  edit hot-applies without dropping connections.
- The loopback management API can replace the whole secret set, so every
  mtg process now gets a random per-process api-token; the manager sends
  it as a bearer token on PUT /secrets and GET /stats and reuses it across
  config rewrites, because mtg reads the token only at startup.
- Malformed tags are rejected at every save path and additionally dropped
  in InstanceFromInbound: one bad tag would otherwise fail the whole
  generated config and take every client of the inbound down with it.
- SyncInbound never copied a re-keyed mtproto secret into the canonical
  clients table, so the clients page and subscription links kept serving
  the old secret, which mtg then rejects. It is now guarded-copied like
  the other credentials.

internal/web/service/inbound.go

@@ -739,6 +739,9 @@ func (s *InboundService) AddInbound(inbound *model.Inbound) (*model.Inbound, boo
 			if client.Secret == "" {
 				return inbound, false, common.NewError("mtproto client requires a secret")
 			}
+			if client.AdTag != "" && !model.ValidMtprotoAdTag(client.AdTag) {
+				return inbound, false, common.NewError("mtproto client ad tag must be 32 hex characters")
+			}
 		default:
 			if client.ID == "" {
 				return inbound, false, common.NewError("empty client ID")