feat: add short IDs to alert results with !alert info command

Each alert result gets a deterministic 8-char base36 ID derived from
backend:item_id. IDs appear in announcements and history, and can be
looked up with !alert info <id> for full details. Existing rows are
backfilled on startup.

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

docs/CHEATSHEET.md

@@ -342,6 +342,7 @@ No API credentials needed (uses public GQL endpoint).
 !alert del <name>                # Remove alert (admin)
 !alert list                      # List alerts
 !alert check <name>              # Force-poll now
+!alert info <id>                 # Show full result details
 !alert history <name> [n]        # Show recent results (default 5)
 ```
 
@@ -350,8 +351,9 @@ Reddit (rd), Mastodon (ft), DuckDuckGo (dg), Google News (gn), Kick (kk),
 Dailymotion (dm), PeerTube (pt), Bluesky (bs), Lemmy (ly), Odysee (od),
 Archive.org (ia), Hacker News (hn), GitHub (gh). Names: lowercase alphanumeric +
 hyphens, 1-20 chars. Keywords: 1-100 chars. Max 20 alerts/channel. Polls every
-5min. Format: `[name/yt] Title -- URL`, etc. No API credentials needed. Persists
-across restarts. History stored in `data/alert_history.db`.
+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`.
 
 ## SearX