Add alert archiving

Alerts can be manually archived (POST /api/alerts/{id}/archive) or
unarchived (DELETE /api/alerts/{id}/archive). A background goroutine
auto-archives resolved alerts older than TERDUT_ARCHIVE_AFTER (default 7d).
GET /api/alerts hides archived alerts by default; ?archived=true shows them.

internal/db/migrations/006_alert_archive.sql

@@ -0,0 +1,2 @@
+ALTER TABLE alerts ADD COLUMN archived_at INTEGER;
+CREATE INDEX alerts_archived_at_idx ON alerts(archived_at);