docs: update for v1.2.1 performance changes

- USAGE.md: alert output format, background seeding, per-backend errors,
  concurrent fetches
- CHEATSHEET.md: updated alert section
- DEBUG.md: added profiling section (cProfile + tracemalloc)
- ROADMAP.md: added v1.2.1 milestone

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/CHEATSHEET.md

@@ -384,9 +384,12 @@ Archive.org (ia), Hacker News (hn), GitHub (gh), Wikipedia (wp),
 Stack Exchange (se), GitLab (gl), npm (nm), PyPI (pp), Docker Hub (dh),
 arXiv (ax), Lobsters (lb), DEV.to (dv), Medium (md), Hugging Face (hf).
 Names: lowercase alphanumeric + hyphens, 1-20 chars. Keywords: 1-100 chars.
-Max 20 alerts/channel. Polls every 5min. Format: `[name/yt/a8k2m] Title -- URL`.
-Use `!alert info <id>` to see full details. No API credentials needed. Persists
-across restarts. History stored in `data/alert_history.db`.
+Max 20 alerts/channel. Polls every 5min. Output: ACTION with `[name/tag/id] date - URL`,
+then PRIVMSG with full title. `add` replies instantly (seeds in background).
+Per-backend error tracking (5+ errors backs off that backend only).
+Multi-instance backends (pt, ft, ly, sx) fetch concurrently.
+Use `!alert info <id>` for details. Persists across restarts.
+History in `data/alert_history.db`.
 
 ## SearX