fix(storage): return guarded unpublish row

[mdorman]

Summary

• make standalone unpublish atomically guard Post owner and live state
• return the complete draft-state PostRecord from backend-parity UPDATE … RETURNING
• remove web-layer pre-read and stale-record replay; derive feed events and SavedPost from the returned row

Verification

• focused dual-backend unpublish lane after rebase
• cargo xtask precommit
• cargo xtask prepush
• final Standards review: no findings
• final Spec review: no findings

Closes #786