feat: unique display names (case-insensitive) + inline recover on join

Backend:
- Migration 007 deduplicates existing users by name (case-insensitive,
  keeping oldest) and adds a unique index on (event_id, LOWER(display_name))
- find_by_event_and_name is now case-insensitive (LOWER comparison)
- join endpoint checks name availability before creating; returns 409
  Conflict with code "conflict" when the name is taken

Frontend (join page):
- On 409, switches to a name-taken view showing an amber warning with
  name-choice tips, a PIN input that runs the recover flow inline, and
  an "Anderen Namen wählen" button that returns to the name input

Test guide: added Steps 8 (rate-limit auto-retry) and 9 (name uniqueness)

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);