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.up.sql

@@ -0,0 +1,15 @@
+-- Deduplicate users with the same name (case-insensitive) per event,
+-- keeping the oldest account so no real data is lost.
+DELETE FROM "user"
+WHERE id NOT IN (
+    SELECT DISTINCT ON (event_id, LOWER(display_name)) id
+    FROM "user"
+    ORDER BY event_id, LOWER(display_name), created_at ASC
+);
+
+-- Drop the old non-unique index (replaced below)
+DROP INDEX IF EXISTS idx_user_event_name;
+
+-- Unique index enforces one account per name per event (case-insensitive)
+CREATE UNIQUE INDEX idx_user_event_name_ci
+    ON "user" (event_id, LOWER(display_name));