Backend: StrEnum Unknown Fallback Pattern

String enum parsing from external/stored values must be resilient.

Rationale:
- New enum values from persisted or external data should not cause uncontrolled runtime failures.

Scope:
- PR review policy for backend runtime changes under `domains/python/backend/app/src/**` involving `StrEnum` and string-to-enum parsing.

Trigger:
- A PR adds/changes a `StrEnum`, or adds/changes coercion from external/persisted string values into enum types.

Approval checks:
1) Enums used for external/persisted inputs define an `UNKNOWN` fallback (or documented equivalent sentinel).
2) Parsing path for external/persisted values handles unknown values explicitly (fallback sentinel or typed validation error at boundary).
3) Direct unchecked coercion on external/persisted values is not introduced.

Evidence:
- Diff shows parsing guard behavior.
- Tests cover at least one unknown-value path for changed enum parsing.

Decision:
- Return NOT APPROVED when trigger is met and resilient parsing checks fail.