Clarify that compact browsing surfaces should receive short post title fields
from the backend even if some frontend pages do not display them yet. Separate
the product requirement from current frontend implementation status so backend
can implement the API contract first and frontend can opt in per surface later.
Add optional post-level and localized title fields, and use a new
postTitleText helper for Resource.title so card/list surfaces prefer short
backend-provided titles instead of full body text. When title is missing,
fallback to the first non-empty body line, then filename, then post id.
Document the backend handoff in docs/posts-title-api.md alongside the other
backend task docs.