perf: harden gateway, billing, and ops paths under high concurrency

[DR-lin-eng]

Summary

• bound OpenAI request and response buffering while improving channel-aware failover behavior
• make usage billing durable under load with queued persistence, bounded fallback, and mutation-version cache guards
• consolidate dashboard reads and add Redis-backed operational and runtime controls

Key changes

• add durable usage billing queueing with synchronous fallback when critical work cannot be enqueued
• prevent stale cache repopulation after billing and balance mutations
• bound request bodies, SSE scanning, session state, OAuth refresh locks, and account runtime state
• improve channel-aware scheduling, retry handling, and temporary account exclusion
• add shared dashboard snapshots and merged usage aggregations
• add Redis-only system logs, runtime retention overrides, user usage statistics, and explicit clear-all operations
• add a global temporary-unschedulable switch with cross-layer cleanup
• expose the new scheduling, logging, retention, and usage controls in the admin UI

Testing

• go test ./...
• golangci-lint v2.9.0 run --timeout=30m ./...
• make test-frontend
• pnpm --dir frontend run build

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13 out of 14 committers have signed the CLA.
✅ (DR-lin-eng)[https://github.com/DR-lin-eng]
✅ (BenjaminAaron196)[https://github.com/BenjaminAaron196]
✅ (yardbirds0)[https://github.com/yardbirds0]
✅ (Eyre921)[https://github.com/Eyre921]
✅ (dftian478)[https://github.com/dftian478]
✅ (SatellaPoi)[https://github.com/SatellaPoi]
✅ (wucm667)[https://github.com/wucm667]
✅ (aomsir)[https://github.com/aomsir]
✅ (adamglin0)[https://github.com/adamglin0]
✅ (feitianbubu)[https://github.com/feitianbubu]
✅ (DylanChiang-Dev)[https://github.com/DylanChiang-Dev]
✅ (AixLau)[https://github.com/AixLau]
✅ (heathermhuang)[https://github.com/heathermhuang]
❌ @CRAZYShimakaze
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