v1.2.1

Add per-experiment "Reset stats" action (v1.2.1)

Clears a single experiment's collected data back to zero without touching
its configuration, for wiping QA traffic before a real run. A "Reset stats"
button in the editor status bar (shown for active and paused experiments)
deletes that experiment's rows from the events and assignments tables, so
every dashboard count returns to zero while variants, metrics, and settings
are kept. Concluded experiments are refused server-side: their frozen
snapshot is the historical record, so clone to re-run instead.

- Repo: reset_stats() deletes events + assignments for one experiment,
  returns the removed counts. Snapshots are left alone.
- Edit: handle_reset_stats() guards on manage_options + nonce (via the
  shared post router) and blocks concluded experiments; new status-bar
  button with a confirm dialog; stats_reset / reset_locked flash notices.
- Admin: route the reset_stats post action.
- Version bumped to 1.2.1 in the header and PBD_EXP_VERSION. No schema
  change, so DB_VERSION stays 1.2.0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>