Make serve a read-only consumer of the on-disk cache

[jdoss]

Summary

Serve and setup each hold their own in-memory dict of the on-disk cache. Without coordination, serve's cache.save() on a lookup miss overwrites setup's freshly pruned state with serve's older dict — resurrecting the stale entries PR #32's prune step just removed.

Observed on the test server immediately after deploying PR #32: cache grew back to 15k+ entries within minutes of setup pruning it down to ~500.

Fix: drop the cache.save() call on the serve cache-miss path. Values still populate the in-memory dict via cache.set(), so subsequent lookups for the same secret in the same process still hit the cache, and existing tests asserting on in-memory presence still pass. The disk file is now owned exclusively by setup, which prunes on every run.

Tradeoff

A secret lazily cached during serve runtime (e.g. an HSM-stored secret outside config.workloads) is lost on serve restart and will miss the cache on its next lookup. Acceptable — the cache's purpose is surviving provider outages for workload secrets, and those are always populated by setup.

Test plan

• pytest tests/test_serve.py tests/test_serve_offline.py tests/test_setup.py — all 38 tests pass, including test_cache_miss_falls_through_to_provider which asserts the in-memory cache still gets populated after a miss.
• ruff check / ty check — clean.
• Deploy to test server, run setup to prune, observe cache size stays at the real secret count instead of growing back to 15k+.