feat(telegram): chat-gate observability + grant on first registration

Two follow-ups from a contour test where a user joined the chat, then
registered, and got no write access — with silent logs.

Observability: log every chat_member update (chat id, configured id, user,
old->new status), the eligibility result and the grant outcome; plus a startup
self-check that warns loudly when the bot is not an administrator in the chat
with the restrict-members ("Ban users") right — the common misconfiguration,
previously invisible in the logs.

Grant on first registration: a user who joins the moderated chat BEFORE
registering is covered by no chat_member event, so the join-time grant never
fires for them. ProvisionTelegram now reports first contact, and the Telegram
auth handler emits chat_access_changed on it, so the gateway re-evaluates and
grants write access if the user is already in the chat.

backend/internal/inttest/account_test.go

@@ -118,10 +118,13 @@ func TestProvisionTelegramSeedsNewAccountOnly(t *testing.T) {
 	store := account.NewStore(testDB)
 	ext := "tg-" + uuid.NewString()
 
-	acc, err := store.ProvisionTelegram(ctx, ext, "ru-RU", "thehandle", "Иван")
+	acc, created, err := store.ProvisionTelegram(ctx, ext, "ru-RU", "thehandle", "Иван")
 	if err != nil {
 		t.Fatalf("provision telegram: %v", err)
 	}
+	if !created {
+		t.Error("created = false on first contact, want true")
+	}
 	if acc.PreferredLanguage != "ru" {
 		t.Errorf("PreferredLanguage = %q, want ru", acc.PreferredLanguage)
 	}
@@ -133,10 +136,13 @@ func TestProvisionTelegramSeedsNewAccountOnly(t *testing.T) {
 	}
 
 	// A later login with different fields returns the same account, unchanged.
-	again, err := store.ProvisionTelegram(ctx, ext, "en", "other", "Other")
+	again, created, err := store.ProvisionTelegram(ctx, ext, "en", "other", "Other")
 	if err != nil {
 		t.Fatalf("re-provision telegram: %v", err)
 	}
+	if created {
+		t.Error("created = true on a repeat login, want false")
+	}
 	if again.ID != acc.ID {
 		t.Errorf("re-provision id = %s, want %s", again.ID, acc.ID)
 	}
@@ -150,7 +156,7 @@ func TestProvisionTelegramSeedsNewAccountOnly(t *testing.T) {
 // language CHECK.
 func TestProvisionTelegramUnknownLanguageDefaults(t *testing.T) {
 	ctx := context.Background()
-	acc, err := account.NewStore(testDB).ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "fr", "", "")
+	acc, _, err := account.NewStore(testDB).ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "fr", "", "")
 	if err != nil {
 		t.Fatalf("provision telegram: %v", err)
 	}
@@ -166,7 +172,7 @@ func TestProvisionTelegramUnknownLanguageDefaults(t *testing.T) {
 func TestHighRateFlagRoundTrip(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
-	acc, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Player")
+	acc, _, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Player")
 	if err != nil {
 		t.Fatalf("provision telegram: %v", err)
 	}
@@ -222,7 +228,7 @@ func TestIdentityExternalID(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
 	ext := "tg-" + uuid.NewString()
-	acc, err := store.ProvisionTelegram(ctx, ext, "en", "", "Tg User")
+	acc, _, err := store.ProvisionTelegram(ctx, ext, "en", "", "Tg User")
 	if err != nil {
 		t.Fatalf("provision telegram: %v", err)
 	}
@@ -247,7 +253,7 @@ func TestIdentityExternalID(t *testing.T) {
 func TestNotificationsInAppOnlyRoundTrip(t *testing.T) {
 	ctx := context.Background()
 	store := account.NewStore(testDB)
-	acc, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Player")
+	acc, _, err := store.ProvisionTelegram(ctx, "tg-"+uuid.NewString(), "en", "", "Player")
 	if err != nil {
 		t.Fatalf("provision telegram: %v", err)
 	}