hotfix: fix Blazor circuit crash on Discord link + add missing avatar_url column

- Replace @onclick button with plain <a href="/auth/discord"> to avoid
circuit disconnect from forceLoad navigation during event handlers.
- Add query param handling (?linked, ?link_error) in Profile.razor for
Discord callback feedback.
- Add V021 migration: ALTER TABLE players ADD COLUMN avatar_url.

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

src/GmRelay.Bot/Migrations/V021__add_avatar_url.sql

@@ -0,0 +1,8 @@
+-- =============================================================
+-- V021: Add avatar_url column to players table
+-- =============================================================
+-- Scope: Support storing avatar URLs for Discord and other platforms.
+-- =============================================================
+
+ALTER TABLE players
+    ADD COLUMN avatar_url VARCHAR(500);