docs(server-changes): scope mollifier drainer-replay note to this PR

Rename the catch-all mollifier.md and trim it to the drainer replay
handler, stale sweep, telemetry gauge, and run-engine cancelled/failed
APIs; later read/mutation/dashboard work is documented in its own PR.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: d-cs

.server-changes/mollifier-drainer-replay.md

@@ -0,0 +1,6 @@
+---
+area: webapp
+type: feature
+---
+
+Mollifier drainer replay: a buffer handler that replays buffered entries into `engine.trigger`, a stale-entry sweep worker, drainer health telemetry (`mollifier.stale_entries.current` gauge), and run-engine APIs to create cancelled and failed runs for drainer-terminal entries.