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/api/archiver.go

@@ -0,0 +1,39 @@
+package api
+
+import (
+	"context"
+	"database/sql"
+	"log"
+	"time"
+)
+
+func StartArchiver(ctx context.Context, db *sql.DB, archiveAfter time.Duration) {
+	ticker := time.NewTicker(15 * time.Minute)
+	defer ticker.Stop()
+
+	run := func() {
+		cutoff := time.Now().Add(-archiveAfter).Unix()
+		res, err := db.ExecContext(ctx,
+			`UPDATE alerts SET archived_at = unixepoch()
+			 WHERE status = 'resolved'
+			   AND archived_at IS NULL
+			   AND COALESCE(ends_at, received_at) < ?`, cutoff)
+		if err != nil {
+			log.Printf("archiver: %v", err)
+			return
+		}
+		if n, _ := res.RowsAffected(); n > 0 {
+			log.Printf("archiver: archived %d resolved alert(s)", n)
+		}
+	}
+
+	run()
+	for {
+		select {
+		case <-ticker.C:
+			run()
+		case <-ctx.Done():
+			return
+		}
+	}
+}