feat(backups): gzip compress backups and add retention config to Instance Settings

Compress database backups with gzip (.sql.gz), reducing file size ~83%.
Add backup retention configuration to Instance Settings UI with preset
options (7 days, 2 weeks, 1 month). The backup scheduler now reads
retention from the database on each tick so changes take effect without
restart. Default retention changed from 30 to 7 days.

packages/shared/src/index.ts

@@ -189,6 +189,7 @@ export type {
   InstanceExperimentalSettings,
   InstanceGeneralSettings,
   InstanceSettings,
+  BackupRetentionDays,
   Agent,
   AgentAccessState,
   AgentChainOfCommandEntry,
@@ -369,6 +370,11 @@ export {
   DEFAULT_FEEDBACK_DATA_SHARING_TERMS_VERSION,
 } from "./types/feedback.js";
 
+export {
+  BACKUP_RETENTION_PRESETS,
+  DEFAULT_BACKUP_RETENTION_DAYS,
+} from "./types/instance.js";
+
 export {
   getClosedIsolatedExecutionWorkspaceMessage,
   isClosedIsolatedExecutionWorkspace,