feat(discord): enable session join leave buttons

Move neutral join/leave handlers into GmRelay.Shared so Telegram and Discord share capacity, waitlist, duplicate-click, and schedule-update behavior.

Add Discord component routing for join_session and leave_session buttons with deferred ephemeral replies and serialized schedule message updates.

Bump version to 2.5.0 and update Discord docs.

Refs #29

compose.yaml

@@ -49,7 +49,7 @@ services:
         crond -f
 
   bot:
-    image: git.codeanddice.ru/toutsu/gmrelay-bot:2.4.0
+    image: git.codeanddice.ru/toutsu/gmrelay-bot:2.5.0
     restart: always
     depends_on:
       db:
@@ -67,7 +67,7 @@ services:
       retries: 3
 
   discord:
-    image: git.codeanddice.ru/toutsu/gmrelay-discord-bot:2.4.0
+    image: git.codeanddice.ru/toutsu/gmrelay-discord-bot:2.5.0
     restart: always
     depends_on:
       db:
@@ -79,7 +79,7 @@ services:
       - gmrelay
 
   web:
-    image: git.codeanddice.ru/toutsu/gmrelay-web:2.4.0
+    image: git.codeanddice.ru/toutsu/gmrelay-web:2.5.0
     restart: always
     depends_on:
       db: