System-prompt-only changelogs now regenerate correctly; period backfill runs inline

Period backfill now runs inline and shows progress — Backfills now create their own sync run and process period windows in parallel chunks (not fire‑and‑forget). The UI shows an "Analyzing changelog entries" status while aggregation runs, and period entries are written before the run is marked idle.

Added a new backfill mode for system-prompt regeneration so you can regenerate all aggregated period summaries with an updated system prompt without touching commit-level summaries.backend
Created aggregating-only sync runs (status 'aggregating') so period-only backfills show progress in the UI while they run.database

Fixed system-prompt-only regeneration so it now triggers the correct backfill event and actually runs (previously the wrong event prevented completion).backend
Fixed the backfill pipeline to process period windows inline (parallel chunks) so aggregation completes reliably and completion status updates correctly.backend
Updated the settings dialog copy to show context-aware wording for system-prompt changes vs provider/model changes so users see accurate guidance before regenerating history.frontend

Improved backfill behavior: when changing frequency we restore archived entries; when regenerating prompts we archive without restoring to force fresh generation with the new prompt.backend
Improved period generation so empty windows are skipped and LLM calls are batched into parallel chunks for efficiency.backend

Added an optional 'mode' field to the changelog/history.backfill.requested event to distinguish frequency changes from prompt-only regeneration.backend
Allowed creating sync runs with an initial status (defaults to 'processing') so callers can create aggregating-only runs.database