feat: unify Telegram and Discord accounts via identity linking

- Add V020 migration: player_links + identity_audit_log tables
- Add ISessionStore methods: ResolveEffectivePlayerId, LinkIdentity, UnlinkIdentity, GetLinkedIdentities
- Update SessionService to resolve effective player id for all permission checks
- Add /auth/discord/callback linking flow when already authenticated
- Add /api/me/identities GET/DELETE endpoints
- Add Profile.razor page for managing linked accounts
- Update NavMenu with profile link and v3.0.0 badge
- Bump version to 3.0.0 across all files

Bump version → 3.0.0

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

compose.yaml

@@ -49,7 +49,7 @@ services:
         crond -f
 
   bot:
-    image: git.codeanddice.ru/toutsu/gmrelay-bot:2.8.1
+    image: git.codeanddice.ru/toutsu/gmrelay-bot:3.0.0
     restart: always
     depends_on:
       db:
@@ -67,7 +67,7 @@ services:
       retries: 3
 
   discord:
-    image: git.codeanddice.ru/toutsu/gmrelay-discord-bot:2.8.1
+    image: git.codeanddice.ru/toutsu/gmrelay-discord-bot:3.0.0
     restart: always
     depends_on:
       db:
@@ -84,7 +84,7 @@ services:
       retries: 3
 
   web:
-    image: git.codeanddice.ru/toutsu/gmrelay-web:2.8.1
+    image: git.codeanddice.ru/toutsu/gmrelay-web:3.0.0
     restart: always
     depends_on:
       db: