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/shared/src/index.ts

@@ -189,7 +189,7 @@ export type {
   InstanceExperimentalSettings,
   InstanceGeneralSettings,
   InstanceSettings,
-  BackupRetentionDays,
+  BackupRetentionPolicy,
   Agent,
   AgentAccessState,
   AgentChainOfCommandEntry,
@@ -371,8 +371,10 @@ export {
 } from "./types/feedback.js";
 
 export {
-  BACKUP_RETENTION_PRESETS,
-  DEFAULT_BACKUP_RETENTION_DAYS,
+  DAILY_RETENTION_PRESETS,
+  WEEKLY_RETENTION_PRESETS,
+  MONTHLY_RETENTION_PRESETS,
+  DEFAULT_BACKUP_RETENTION,
 } from "./types/instance.js";
 
 export {