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-lib.test.ts

@@ -125,7 +125,7 @@ describeEmbeddedPostgres("runDatabaseBackup", () => {
         const result = await runDatabaseBackup({
           connectionString: sourceConnectionString,
           backupDir,
-          retentionDays: 7,
+          retention: { dailyDays: 7, weeklyWeeks: 4, monthlyMonths: 1 },
           filenamePrefix: "paperclip-test",
         });