feat: add flaskpaste plugin with paste/shorten commands

- PoW-authenticated paste creation and URL shortening via FlaskPaste
- !paste <text> creates a paste, !shorten <url> shortens a URL
- Module-level shorten_url/create_paste helpers for cross-plugin use
- Alert plugin auto-shortens URLs in announcements and history output
- Custom TLS CA cert support via secrets/flaskpaste/derp.crt
- No SOCKS proxy -- direct urllib.request to FlaskPaste instance

docs/CHEATSHEET.md

@@ -273,6 +273,16 @@ API keys: set `ABUSEIPDB_API_KEY` / `VIRUSTOTAL_API_KEY` env vars or
 configure in `config/derp.toml` under `[abuseipdb]` / `[virustotal]`.
 VT rate limit: 4 req/min. Email check: max 5, admin only.
 
+## FlaskPaste
+
+```
+!paste some text here             # Create paste, get URL
+!shorten https://long-url.com/x   # Shorten URL
+```
+
+Auto-shortens URLs in `!alert` announcements and history when plugin is loaded.
+Config: `[flaskpaste]` in `config/derp.toml` or `FLASKPASTE_URL` env var.
+
 ### Data Setup
 
 ```bash