feat: add Eventify ID (EVT-XXXXXXXX) to User model and all APIs

- Add eventify_id CharField (unique, indexed, editable=False) to User
- Auto-generate on save() with charset excluding I/O/0/1 for clarity
- Migration 0012: add field nullable, backfill all existing users, make non-null
- Sync migration 0011 (allowed_modules) pulled from server
- Expose eventify_id in accounts/api.py, partner/api.py serializers
- Expose eventify_id in mobile_api login response (populates localStorage)

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

accounts/api.py

@@ -17,6 +17,7 @@ def _partner_user_to_dict(user, request=None):
         user,
         fields=[
             "id",
+            "eventify_id",
             "username",
             "email",
             "phone_number",