Regenerate period history by deleting old summaries; feeds now exclude commit entries

Settings changes now regenerate period summaries from scratch — When you change aggregation schedule or LLM settings, all daily/weekly/monthly summaries are now hard-deleted and rebuilt so new entries reflect the current settings. Commit-level entries are not affected.
Public feeds and discovery now show only aggregated changelog entries — RSS, Atom, sitemap and the LLM list now exclude commit-level entries and only include daily/weekly/monthly summaries to avoid exposing commit noise in public feeds.

Hard-deleted all period (daily/weekly/monthly) changelog entries when settings changed so regenerated history always uses the latest LLM and aggregation configuration; commit entries were left untouched.backend

Removed the manual Resync button and the per-repository sync status badge from the project overview to simplify the UI and avoid duplicate indicators.frontend
Replaced archive-and-restore backfill flow with a single delete step so period entries are always generated fresh with current settings.backend
Filtered public feeds (RSS, Atom, sitemap, llms.txt) to include only aggregated entry types (daily/weekly/monthly), excluding commit-level entries from public discovery surfaces.frontend

Removed the obsolete 'mode' field from the changelog/history.backfill.requested event and from the settings route payload; backfills now always delete and regenerate period entries.backend

Added a repository operation to hard-delete period entries by project (daily/weekly/monthly) used during settings-driven regeneration.database