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/userlist_test.go

@@ -18,7 +18,7 @@ func TestUserListFilter(t *testing.T) {
 	st := account.NewStore(testDB)
 	uniq := uuid.NewString()
 
-	human, err := st.ProvisionTelegram(ctx, "tg-"+uniq, "en", "", "Zzqxhuman")
+	human, _, err := st.ProvisionTelegram(ctx, "tg-"+uniq, "en", "", "Zzqxhuman")
 	if err != nil {
 		t.Fatalf("provision human: %v", err)
 	}