perf(api): count session posts without loading bodies

[benvinegar]

Summary

• count posts for GET /api/sessions through an optional narrow store capability
• use one SQLite GROUP BY query without selecting or decoding surface/history bodies
• avoid cloning and sorting posts in the loaded JSON store
• preserve compatibility with custom stores through the existing listPosts() fallback
• cover optimized dispatch, fallback behavior, both built-in stores, mutations, and exact SQL shape

Performance

A local synthetic 10,000-post SQLite probe measured approximately 32 ms for the previous full-post load versus 3.3 ms for the aggregate path (~9.7×). This is directional local evidence, not a CI benchmark.

Validation

• npm test
• npm run coverage
• npm run test:worker
• npm run typecheck
• npm run lint
• npm run format:check
• focused API and store suites

npm run security:audit currently reports dependency advisories already present on main.

This PR description was generated by Pi using gpt-5.6-sol