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>

spec.md

@@ -156,10 +156,11 @@
 |--------|------|-------------|
 | id | INT | PRIMARY KEY AUTO_INCREMENT |
 | event_id | INT | NOT NULL, FOREIGN KEY → events(id) |
-| 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 |
+| | | UNIQUE KEY (event_id, code) per event |
 
 ## 4. User Types & Roles