monitor_cache — JSON bytes storage (no more deepcopy)

Motivation

Every cache hit in cached_snapshot paid a copy.deepcopy(value) — _refresh deepcopied the loaded payload before storing, and _with_cache_meta deepcopied again on every read. AKS monitor payloads (nodes/pods/jobs) on a cluster of any size are tens of KB to MB; the SPA polls 6 monitor routes every few seconds, so this was one of the most expensive paths on the api sidecar event loop's thread pool.

User-facing change

None. Same dict shape returned; serialization roundtrip yields a fresh mutable dict (same isolation as deepcopy) at a fraction of the cost.

API / IaC diff

• api/services/monitor_cache.py
• _SnapshotEntry.value (dict) → _SnapshotEntry.payload_bytes (bytes). _refresh does json.dumps(payload, default=str) once and stores the encoded bytes.
• _with_cache_meta takes bytes, does json.loads once, then inserts the cache-meta dict. Behaves identically when the loader returned a non-dict value (wrapped under {"value": …}).
• default=str keeps datetime / UUID values from raising during serialization — matches the previous deepcopy's tolerance for arbitrary value types.
• All four entry.value / fallback.value / refreshed.value / loader() call sites updated to pass bytes.

Validation

• uv run pytest -q api/tests/test_monitor_cache.py api/tests/test_warmup_route.py — 20 passed.
• uv run ruff check api/services/monitor_cache.py — clean.