[Feature]: Fix delivery pipeline — DELETE handling + one-per-record migration

Parent PRD

#254

What to build

End-to-end fix for the indirect delivery pipeline covering three concerns:

1. File model migration: Switch DataDomainFileModel, RoleFileModel, and TagFileModel from ONE_FILE_PER_RECORD=False (single shared file) to ONE_FILE_PER_RECORD=True (individual files per record). Each gets a proper get_filename(entity) returning a sanitized name like hr-domain.yaml, admin-role.yaml, classification-namespace.yaml. The serialize_all code path in FileModel base is retained but unused.

2. DELETE handling in _deliver_indirect: Detect *_DELETE change types and call git_service.delete_file() with the correct filename and subdirectory for the entity type. For unknown entity types (no file model), delete the raw change file.

3. Manager DELETE wiring: Add _queue_delivery(entity, DeliveryChangeType.*_DELETE, ...) calls in the delete methods of all 5 existing delivery-enabled managers (DataProducts, DataContracts, Datasets, DataDomains, Tags). Refactor SettingsManager inline delivery logic for AppRoles (~lines 1770-1824) to inherit DeliveryMixin instead.

4. Unit tests: Test CREATE writes a YAML file, UPDATE overwrites the correct file, DELETE removes the correct file, unknown entity type falls back to raw change data — all via mock GitService.

Acceptance criteria

• DataDomainFileModel, RoleFileModel, TagFileModel use ONE_FILE_PER_RECORD = True with per-entity filenames
• _deliver_indirect handles *_DELETE change types by calling delete_file()
• All 5 existing delivery-enabled managers call _queue_delivery on delete operations
• SettingsManager uses DeliveryMixin for AppRole delivery (inline logic removed)
• Unit tests cover CREATE, UPDATE, DELETE, and unknown-entity-type fallback paths

Blocked by

None — can start immediately.

User stories addressed

• User story 4 (deleted contracts removed from Git)
• User story 11 (domain changes don't clobber each other)
• User story 14 (product deletions reflected in Git)
• User story 15 (role and tag changes don't clobber)
• User story 18 (K8s-style resource format maintained)

Start by tracing FileModel and the DataDomainFileModel, RoleFileModel, and TagFileModel implementations, then follow _deliver_indirect and the DeliveryMixin queue calls. Review delete methods in the five named managers and the SettingsManager AppRoles logic around lines 1770-1824. Done means per-record filenames, correct DELETE behavior and fallback handling, migrated manager wiring, and mock GitService tests for CREATE, UPDATE, DELETE, and unknown entities.