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/src/auth/handlers.rs

@@ -57,6 +57,14 @@ pub async fn join(
     )
     .await?;
 
+    // Reject if a user with this name (case-insensitive) already exists
+    if User::name_taken(&state.pool, event.id, display_name).await? {
+        return Err(AppError::Conflict(format!(
+            "Der Name \"{}\" ist bereits vergeben.",
+            display_name
+        )));
+    }
+
     // Generate a 4-digit PIN
     let pin: String = format!("{:04}", rand::rng().random_range(0..10000u32));
     let pin_hash =

backend/src/models/user.rs

@@ -59,7 +59,7 @@ impl User {
         display_name: &str,
     ) -> Result<Vec<Self>, sqlx::Error> {
         sqlx::query_as::<_, Self>(
-            "SELECT * FROM \"user\" WHERE event_id = $1 AND display_name = $2",
+            "SELECT * FROM \"user\" WHERE event_id = $1 AND LOWER(display_name) = LOWER($2)",
         )
         .bind(event_id)
         .bind(display_name)
@@ -67,6 +67,21 @@ impl User {
         .await
     }
 
+    pub async fn name_taken(
+        pool: &PgPool,
+        event_id: Uuid,
+        display_name: &str,
+    ) -> Result<bool, sqlx::Error> {
+        let row: (bool,) = sqlx::query_as(
+            "SELECT EXISTS(SELECT 1 FROM \"user\" WHERE event_id = $1 AND LOWER(display_name) = LOWER($2))",
+        )
+        .bind(event_id)
+        .bind(display_name)
+        .fetch_one(pool)
+        .await?;
+        Ok(row.0)
+    }
+
     pub async fn increment_failed_pin(pool: &PgPool, id: Uuid) -> Result<i16, sqlx::Error> {
         let row: (i16,) = sqlx::query_as(
             "UPDATE \"user\"