feat: persist alert results to SQLite history table

Matched results were announced then discarded. Add a dedicated SQLite
database (data/alert_history.db) to store every announced result with
channel, alert name, backend, title, URL, date, and timestamp. Add
!alert history <name> [n] subcommand to query recent results.

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

docs/USAGE.md

@@ -123,6 +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 |
 | `!searx <query>` | Search SearXNG and show top results |
 
 ### Command Shorthand
@@ -666,6 +667,7 @@ simultaneously.
 !alert del <name>                Remove alert (admin)
 !alert list                      List alerts
 !alert check <name>              Force-poll now
+!alert history <name> [n]        Show recent results (default 5, max 20)
 ```
 
 - `add` and `del` require admin privileges
@@ -692,5 +694,7 @@ Polling and announcements:
 - Each platform maintains its own seen list (capped at 200 per platform)
 - 5 consecutive errors doubles the poll interval (max 1 hour)
 - Subscriptions persist across bot restarts via `bot.state`
+- Matched results are stored in `data/alert_history.db` (SQLite)
 - `list` shows error status indicators next to each alert
 - `check` forces an immediate poll across all platforms
+- `history` queries stored results, most recent first