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.

server/src/index.ts

@@ -642,12 +642,12 @@ export async function startServer(): Promise<StartedServer> {
       try {
         // Read retention from Instance Settings (DB) so changes take effect without restart
         const generalSettings = await settingsSvc.getGeneral();
-        const retentionDays = generalSettings.backupRetentionDays;
+        const retention = generalSettings.backupRetention;
 
         const result = await runDatabaseBackup({
           connectionString: activeDatabaseConnectionString,
           backupDir: config.databaseBackupDir,
-          retentionDays,
+          retention,
           filenamePrefix: "paperclip",
         });
         logger.info(
@@ -656,7 +656,7 @@ export async function startServer(): Promise<StartedServer> {
             sizeBytes: result.sizeBytes,
             prunedCount: result.prunedCount,
             backupDir: config.databaseBackupDir,
-            retentionDays,
+            retention,
           },
           `Automatic database backup complete: ${formatDatabaseBackupResult(result)}`,
         );