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>

src/GmRelay.Web/Services/ISessionStore.cs

@@ -53,4 +53,19 @@ public interface ISessionStore
     Task LogSessionChangeAsync(Guid sessionId, string actorExternalUserId, string actorName, string changeType, string? oldValue, string? newValue);
     Task<List<SessionAuditLogEntry>> GetSessionHistoryAsync(Guid sessionId);
     Task UpsertDiscordUserAsync(string discordId, string displayName, string? avatarUrl);
+
+    // --- Identity linking (issue #35) ---
+    Task<Guid?> ResolveEffectivePlayerIdAsync(string platform, string externalUserId);
+    Task<List<LinkedIdentity>> GetLinkedIdentitiesAsync(string platform, string externalUserId);
+    Task LinkIdentityAsync(string currentPlatform, string currentExternalUserId, string targetPlatform, string targetExternalUserId, string? currentName);
+    Task UnlinkIdentityAsync(string currentPlatform, string currentExternalUserId, string targetPlatform, string targetExternalUserId);
+    Task UpsertPlayerAsync(string platform, string externalUserId, string displayName, string? avatarUrl);
 }
+
+public sealed record LinkedIdentity(
+    string Platform,
+    string ExternalUserId,
+    string DisplayName,
+    string? ExternalUsername,
+    string? AvatarUrl,
+    DateTime LinkedAt);