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.

cli/src/commands/db-backup.ts

@@ -73,7 +73,7 @@ export async function dbBackupCommand(opts: DbBackupOptions): Promise<void> {
     const result = await runDatabaseBackup({
       connectionString: connection.value,
       backupDir,
-      retentionDays,
+      retention: { dailyDays: retentionDays, weeklyWeeks: 4, monthlyMonths: 1 },
       filenamePrefix,
     });
     spinner.stop(`Backup saved: ${formatDatabaseBackupResult(result)}`);

cli/src/commands/worktree.ts

@@ -903,7 +903,7 @@ async function seedWorktreeDatabase(input: {
     const backup = await runDatabaseBackup({
       connectionString: sourceConnectionString,
       backupDir: path.resolve(input.targetPaths.backupDir, "seed"),
-      retentionDays: 7,
+      retention: { dailyDays: 7, weeklyWeeks: 4, monthlyMonths: 1 },
       filenamePrefix: `${input.instanceId}-seed`,
       includeMigrationJournal: true,
       excludeTables: seedPlan.excludedTables,