feat(telegram): promo bot + channel-chat moderation gate

Add a second standalone promo bot to the bot container (answers /start with a
localized message + a URL button into the main bot's Mini App) and gate write
access in a channel's linked discussion chat: grant on join when the Telegram
user is registered and neither admin-suspended nor holding a new chat_muted
role, and revoke/grant on the matching moderation change for a member currently
in the chat.

Eligibility (registered AND NOT suspended AND NOT chat_muted; the game
suspension dominates) is resolved once in the backend and reached two ways: the
bot's join-time unary ResolveChatEligibility over the existing mTLS bot-link,
and a backend chat_access_changed event -> gateway -> ChatGate command
(idempotent; a temporary-block-expiry sweeper may over-emit). The bot guards the
block/unblock path with getChatMember, since bots cannot list members.

A web_app button cannot open another bot's Mini App (it signs initData with the
sending bot's token), so the promo button is a t.me ?startapp URL reusing the
UI's VITE_TELEGRAM_LINK. The bot must be a chat admin with the restrict-members
right and chat_member in its allowed updates.

No schema change: chat_muted reuses the data-driven account_roles table.

gateway/internal/backendclient/api.go

@@ -215,6 +215,34 @@ func (c *Client) PushTarget(ctx context.Context, userID string) (PushTargetResp,
 	return out, err
 }
 
+// ChatAccessResp is a user's moderated-chat write eligibility: ExternalID is their
+// Telegram identity (empty when they have none, so the gateway has nothing to gate),
+// Registered whether an account was found, and Eligible the final gate the bot applies
+// (registered and neither admin-suspended nor chat-muted).
+type ChatAccessResp struct {
+	ExternalID string `json:"external_id"`
+	Registered bool   `json:"registered"`
+	Eligible   bool   `json:"eligible"`
+}
+
+// ChatEligibility resolves a Telegram identity to its moderated-chat write
+// eligibility — the join path, when the bot sees a user enter the chat.
+func (c *Client) ChatEligibility(ctx context.Context, externalID string) (ChatAccessResp, error) {
+	var out ChatAccessResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/internal/chat-access", "", "",
+		map[string]string{"external_id": externalID}, &out)
+	return out, err
+}
+
+// ChatAccessByUser resolves an account id to its Telegram identity and current
+// moderated-chat write eligibility — the change path, for a chat-access-changed event.
+func (c *Client) ChatAccessByUser(ctx context.Context, userID string) (ChatAccessResp, error) {
+	var out ChatAccessResp
+	err := c.do(ctx, http.MethodPost, "/api/v1/internal/chat-access", "", "",
+		map[string]string{"user_id": userID}, &out)
+	return out, err
+}
+
 // GuestAuth provisions a guest account and mints a session.
 func (c *Client) GuestAuth(ctx context.Context) (SessionResp, error) {
 	var out SessionResp