Fix attendee_types.code uniqueness: per-event not global

Also update generate_type_code() to accept event_id parameter
for proper per-event uniqueness checking.

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

init_db.py

@@ -189,11 +189,12 @@ def create_tables():
         CREATE TABLE IF NOT EXISTS attendee_types (
             id INT PRIMARY KEY AUTO_INCREMENT,
             event_id INT NOT NULL,
-            code VARCHAR(10) UNIQUE NOT NULL,
+            code VARCHAR(10) NOT NULL,
             name VARCHAR(100) NOT NULL,
             price DECIMAL(10,2) DEFAULT 0.00,
             created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
-            FOREIGN KEY (event_id) REFERENCES events(id) ON DELETE CASCADE
+            FOREIGN KEY (event_id) REFERENCES events(id) ON DELETE CASCADE,
+            UNIQUE KEY unique_type_code_per_event (event_id, code)
         )
         """
     ]