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/mtproto/manager_mutation_test.go

@@ -30,6 +30,10 @@ func TestScrapeStats(t *testing.T) {
 			http.NotFound(w, r)
 			return
 		}
+		if r.Header.Get("Authorization") != "Bearer sesame" {
+			w.WriteHeader(http.StatusUnauthorized)
+			return
+		}
 		_, _ = io.WriteString(w, `{"started_at":"2026-01-01T00:00:00Z","total_connections":2,`+
 			`"users":{`+
 			`"alice":{"connections":2,"bytes_in":100,"bytes_out":200,"last_seen":"2026-01-01T00:01:00Z"},`+
@@ -37,7 +41,7 @@ func TestScrapeStats(t *testing.T) {
 	}))
 	defer srv.Close()
 
-	users, ok := scrapeStats(serverPort(t, srv))
+	users, ok := scrapeStats(serverPort(t, srv), "sesame")
 	if !ok {
 		t.Fatal("scrapeStats should succeed against a valid /stats endpoint")
 	}
@@ -59,7 +63,7 @@ func TestScrapeStatsUnreachable(t *testing.T) {
 	port := serverPort(t, srv)
 	srv.Close()
 
-	if _, ok := scrapeStats(port); ok {
+	if _, ok := scrapeStats(port, ""); ok {
 		t.Fatal("scrapeStats must report ok=false when the endpoint is unreachable")
 	}
 }