feat(backups): tiered daily/weekly/monthly retention with UI controls

Replace single retentionDays with a three-tier BackupRetentionPolicy:
- Daily: keep all backups (presets: 3, 7, 14 days; default 7)
- Weekly: keep one per calendar week (presets: 1, 2, 4 weeks; default 4)
- Monthly: keep one per calendar month (presets: 1, 3, 6 months; default 1)

Pruning sorts backups newest-first and applies each tier's cutoff,
keeping only the newest entry per ISO week/month bucket. The Instance
Settings General page now shows three preset selectors (no icon, matches
existing page design). Remove Database icon import.

packages/db/src/backup.ts

@@ -85,7 +85,7 @@ function resolveBackupDir(config: PartialConfig | null): string {
 }
 
 function resolveRetentionDays(config: PartialConfig | null): number {
-  return asPositiveInt(config?.database?.backup?.retentionDays) ?? 30;
+  return asPositiveInt(config?.database?.backup?.retentionDays) ?? 7;
 }
 
 async function main() {
@@ -103,7 +103,7 @@ async function main() {
     const result = await runDatabaseBackup({
       connectionString,
       backupDir,
-      retentionDays,
+      retention: { dailyDays: retentionDays, weeklyWeeks: 4, monthlyMonths: 1 },
       filenamePrefix: "paperclip",
     });