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/USAGE.md

@@ -123,7 +123,7 @@ format = "text"               # Log format: "text" (default) or "json"
 | `!username <user>` | Check username across ~25 services |
 | `!username <user> <service>` | Check single service |
 | `!username list` | Show available services by category |
-| `!alert <add\|del\|list\|check\|history>` | Keyword alert subscriptions across platforms |
+| `!alert <add\|del\|list\|check\|info\|history>` | Keyword alert subscriptions across platforms |
 | `!searx <query>` | Search SearXNG and show top results |
 
 ### Command Shorthand
@@ -670,6 +670,7 @@ supported platforms simultaneously.
 !alert del <name>                Remove alert (admin)
 !alert list                      List alerts
 !alert check <name>              Force-poll now
+!alert info <id>                 Show full details for a result
 !alert history <name> [n]        Show recent results (default 5, max 20)
 ```
 
@@ -703,9 +704,9 @@ Polling and announcements:
 
 - Alerts are polled every 5 minutes by default
 - On `add`, existing results are recorded without announcing (prevents flood)
-- New results announced as `[name/<tag>] Title -- URL` where tag is one of:
+- New results announced as `[name/<tag>/<id>] Title -- URL` where tag is one of:
   `yt`, `tw`, `sx`, `rd`, `ft`, `dg`, `gn`, `kk`, `dm`, `pt`, `bs`, `ly`, `od`, `ia`,
-  `hn`, `gh`
+  `hn`, `gh` and `<id>` is a short deterministic ID for use with `!alert info`
 - Titles are truncated to 80 characters
 - Each platform maintains its own seen list (capped at 200 per platform)
 - 5 consecutive errors doubles the poll interval (max 1 hour)