Every authenticated API request previously called config_mgr.set(api_keys),
which rewrites the entire app config blob to SQLite. Under load this caused
significant write amplification and lock contention.
Persistence of last_used_at is now throttled per key hash to once every
15 minutes. The in-memory timestamp on the matched key is still updated
immediately, so reads within the same process see the live value; only
the on-disk persistence is throttled.
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