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/client_crud.go

@@ -435,6 +435,15 @@ func (s *ClientService) Update(inboundSvc *InboundService, id int, updated model
 		return needRestart, err
 	}
 
+	// Same shape as the group write above: SyncInbound keeps a stored ad-tag
+	// when the incoming settings carry none, so clearing the override must be
+	// applied here, where the editor always round-trips the field.
+	if err := database.GetDB().Model(&model.ClientRecord{}).
+		Where("id = ?", id).
+		UpdateColumn("ad_tag", updated.AdTag).Error; err != nil {
+		return needRestart, err
+	}
+
 	if err := database.GetDB().Model(&model.ClientRecord{}).
 		Where("id = ?", id).
 		UpdateColumn("enable", updated.Enable).Error; err != nil {