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>

tests/GmRelay.Bot.Tests/Web/AuthorizedSessionServiceTests.cs

@@ -1121,6 +1121,21 @@ public sealed class AuthorizedSessionServiceTests
         public Task UpsertDiscordUserAsync(string discordId, string displayName, string? avatarUrl) =>
             Task.CompletedTask;
 
+        public Task<Guid?> ResolveEffectivePlayerIdAsync(string platform, string externalUserId) =>
+            Task.FromResult<Guid?>(Guid.NewGuid());
+
+        public Task<List<LinkedIdentity>> GetLinkedIdentitiesAsync(string platform, string externalUserId) =>
+            Task.FromResult(new List<LinkedIdentity>());
+
+        public Task LinkIdentityAsync(string currentPlatform, string currentExternalUserId, string targetPlatform, string targetExternalUserId, string? currentName) =>
+            Task.CompletedTask;
+
+        public Task UnlinkIdentityAsync(string currentPlatform, string currentExternalUserId, string targetPlatform, string targetExternalUserId) =>
+            Task.CompletedTask;
+
+        public Task UpsertPlayerAsync(string platform, string externalUserId, string displayName, string? avatarUrl) =>
+            Task.CompletedTask;
+
         private bool IsManager(Guid groupId, long telegramId) =>
             IsOwner(groupId, telegramId) ||
             managers.Any(manager => manager.GroupId == groupId && manager.TelegramId == telegramId);