perf(scheduler): strip id_token and refresh_token from scheduler cache

[Fumeng24]

Summary

The scheduler cache (sched:acc:{id}) stores full Account JSON blobs in Redis, including large OAuth tokens (id_token, refresh_token). In multi-instance deployments with limited inter-server bandwidth (e.g. 30Mbps), this causes excessive Redis traffic:

• Each account entry is ~20KB due to JWT tokens
• With 1000 accounts, a full rebuild writes ~20MB to Redis
• Observed ~19.5Mbps of Redis upload traffic from scheduler cache writes alone

The gateway request path only needs access_token and api_key. The id_token and refresh_token are consumed exclusively by background token-refresh services that read directly from the database.

Changes

• Add marshalAccountForCache() helper that strips id_token and refresh_token before serialization, using a shallow copy to avoid mutating the caller's data
• Apply to SetAccount, SetSnapshot, and UpdateLastUsed write paths
• Add unit tests for the stripping logic (nil account, empty credentials, strip verification, mutation safety)

Impact

• ~60-70% reduction in per-account Redis payload size
• Significantly reduces cross-server Redis bandwidth in multi-instance deployments
• No impact on gateway functionality — access_token and api_key are preserved
• No impact on token refresh — refresh services read credentials from PostgreSQL

Test plan

• Unit tests for marshalAccountForCache (nil, empty, strip, no-mutation)
• Verify gateway requests still work (access_token/api_key preserved in cache)
• Verify token refresh still works (reads from DB, not cache)
• Monitor Redis bandwidth reduction with iftop -f "port 6379"