feat: unique display names + inline recover on join (v0.13.1)

Backend: migration 007 adds a case-insensitive unique index on user names
per event. join endpoint returns 409 conflict when the name is taken.
find_by_event_and_name uses LOWER() for case-insensitive recovery.

Frontend: join page handles 409 with a name-taken view — amber warning,
name-choice tips, inline PIN recovery form, and "Anderen Namen wählen"
button. Test guide updated with Steps 8 and 9.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/007_user_name_unique.down.sql

@@ -0,0 +1,4 @@
+DROP INDEX IF EXISTS idx_user_event_name_ci;
+
+CREATE INDEX idx_user_event_name
+    ON "user"(event_id, display_name);