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/config/config.go

@@ -1,10 +1,14 @@
 package config
 
-import "os"
+import (
+	"os"
+	"time"
+)
 
 type Config struct {
-	Addr   string
-	DBPath string
+	Addr         string
+	DBPath       string
+	ArchiveAfter time.Duration
 }
 
 func Load() Config {
@@ -16,5 +20,11 @@ func Load() Config {
 	if dbPath == "" {
 		dbPath = "terdut.db"
 	}
-	return Config{Addr: addr, DBPath: dbPath}
+	archiveAfter := 7 * 24 * time.Hour
+	if s := os.Getenv("TERDUT_ARCHIVE_AFTER"); s != "" {
+		if d, err := time.ParseDuration(s); err == nil {
+			archiveAfter = d
+		}
+	}
+	return Config{Addr: addr, DBPath: dbPath, ArchiveAfter: archiveAfter}
 }