fix: combine session image and text into single Telegram message

When creating a session with an image, send it as a single SendPhoto
with the schedule text as caption (+ reply markup), instead of two
separate messages. Falls back to two messages if caption exceeds
Telegram's 1024-char limit.

Also adds BatchMessageEditor helper that transparently handles
EditMessageText vs EditMessageCaption depending on whether the batch
message is a text or photo message. Updated all handlers and web
service to use this helper.

Version bump to 1.9.7.

src/GmRelay.Web/Services/SessionService.cs

@@ -1048,11 +1048,11 @@ public sealed class SessionService(
 
             var renderResult = SessionBatchRenderer.Render(title, sessions, participants);
 
-            await bot.EditMessageText(
+            await BatchMessageEditor.EditBatchMessageAsync(
+                bot,
                 chatId: chatId,
                 messageId: messageId,
                 text: renderResult.Text,
-                parseMode: Telegram.Bot.Types.Enums.ParseMode.Html,
                 replyMarkup: renderResult.Markup);
         }
         catch (Exception ex)