feat(scheduler): improve account routing and visibility

[laozuzhen]

Summary

• Add account-level scheduling metadata and context-length filters for account selection.
• Improve OpenAI account scheduler/model routing eligibility and cache behavior.
• Add admin account UI visibility for recent/account stats and routing-related fields.

Tests

• go test ./internal/service ./internal/repository ./internal/handler/admin -run 'Account|Scheduler|OpenAI|ContextLength|Routing|AvailableModels|SyncUpstreamModels'

Notes

• Frontend build was not run in the isolated worktree because frontend/node_modules is not installed there.

[github-actions]

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[laozuzhen]

recheck