perf(question-feed): cache get_remaining_estimate with 300s TTL on shared cache + schedule dedupe migration

[WilfordGrimley]

Summary

1. Cache get_remaining_estimate: Wrapped body counts in caches["shared"] with 300s TTL.
2. DatabaseCache Substrate: Landed shared DB cache backend.
3. Schedule Dedupe Migration: Added dedupe migration and unique schedule name constraint.

Test plan

• Python unit tests pass.
• Linting checks pass.
• CI — not watched by this session per dispatch convention