feat: implement /api/update/readiness endpoint + structured update ch…

[eva57gr]

…ecks

[Lightheartdevs]

Review: REQUEST CHANGES

Blocking: Bare except Exception in _check_compatibility_status()

CLAUDE.md: "Never except Exception: return." This catches everything and returns a dict instead of crashing. Narrow to except OSError (the only realistic failure mode beyond the already-caught TimeoutExpired), or remove.

Blocking: Synchronous subprocess.run in async endpoint

Blocks the event loop for up to 30s. Every other subprocess call in this file uses asyncio.create_subprocess_exec. Use that or asyncio.to_thread().

Blocking: Duplicated script resolution

New _resolve_update_script_for_readiness() has a different candidate list than trigger_update(). Readiness could report "available" while POST /api/update returns 501. Unify into one shared function.

Non-blocking:

• Dead model_dump / isinstance coercion block — get_version() always returns dict, remove the duck-typing
• datetime.now(timezone.utc).isoformat() + "Z" repeated 6+ times — extract helper
• Tests are well-structured, good coverage

🤖 Reviewed with Claude Code