Deprecate EnumValueType/EnumType in src/ai/backend/manager/models/base.py and standardize on StrEnumType

Background
src/ai/backend/manager/models/base.py currently contains EnumValueType, EnumType, and StrEnumType.

Goal
Deprecate EnumValueType and EnumType, and standardize enum handling on StrEnumType only.

Scope
1. Define and implement deprecation policy for EnumValueType and EnumType
2. Migrate existing usages in the manager codebase to StrEnumType incrementally
3. Update related tests, typing, and documentation
4. Document removal timeline and backward\-compatibility guidance

Definition of Done
\- New code does not use EnumValueType or EnumType
\- Existing usages are migrated or have a clear migration plan
\- CI and quality checks pass
\- Changes and migration guidance are documented

JIRA Issue: BA-4703