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/DEBUG.md

@@ -97,6 +97,41 @@ ERROR derp.plugin  failed to load plugin: plugins/broken.py
 - In container: resolver is typically `127.0.0.11` (Podman DNS)
 - Fallback: `8.8.8.8` if no resolver found in `/etc/resolv.conf`
 
+## Profiling
+
+### CPU (cProfile)
+
+```bash
+derp --cprofile                     # Dump to derp.prof on shutdown
+derp --cprofile /app/data/derp.prof # Custom path
+```
+
+Analyze with:
+
+```python
+import pstats
+p = pstats.Stats("data/derp.prof")
+p.sort_stats("tottime").print_stats(30)
+p.sort_stats("cumulative").print_stats("plugins/", 20)
+```
+
+### Memory (tracemalloc)
+
+```bash
+derp --tracemalloc                  # 10 frames (default)
+derp --tracemalloc 25               # 25 frames deep
+```
+
+Writes top 25 allocations with full tracebacks to `data/derp.malloc`
+on clean shutdown. Both flags can be combined:
+
+```bash
+derp --verbose --cprofile /app/data/derp.prof --tracemalloc
+```
+
+Requires clean SIGTERM shutdown (not SIGKILL) to flush data.
+Use `podman stop -t 30 derp` to allow graceful shutdown.
+
 ## Testing IRC Connection
 
 ```bash