fix: Discord OAuth CSRF cookie SameSite=None for cross-site callback

- Changed __DiscordOAuthState cookie from SameSite=Strict to SameSite=None
  because Discord redirects from discord.com (cross-site) and Strict
  prevents the cookie from being sent on the callback request.
- Added logging for CSRF validation failure to aid future diagnostics.

Bump version → 2.8.1

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

compose.yaml

@@ -67,7 +67,7 @@ services:
       retries: 3
 
   discord:
-    image: git.codeanddice.ru/toutsu/gmrelay-discord-bot:2.8.0
+    image: git.codeanddice.ru/toutsu/gmrelay-discord-bot:2.8.1
     restart: always
     depends_on:
       db:
@@ -84,7 +84,7 @@ services:
       retries: 3
 
   web:
-    image: git.codeanddice.ru/toutsu/gmrelay-web:2.8.0
+    image: git.codeanddice.ru/toutsu/gmrelay-web:2.8.1
     restart: always
     depends_on:
       db: