[codex] Add manual scheduler candidate index

[AzurCrystal]

Summary

• add manual scheduler candidate indexes for selected large buckets
• read only a bounded candidate set before falling back to the full scheduler snapshot
• add config wiring and tests for indexed bucket behavior and fallback safety

Why

Large OpenAI account pools were forcing hot request paths to materialize and JSON-decode full Redis scheduler snapshots. This adds an opt-in path so specific buckets can be sampled through a Redis candidate index instead of reading the entire pool on every load-balance selection.

Validation

• go test ./backend/internal/service ./backend/internal/repository