fix: prevent excessive disk usage from repo backups (#235)

* fix: prevent excessive disk usage from repo backups (#234)

Legacy configs with backupBeforeSync: true but no explicit backupStrategy
silently resolved to "always", creating full git bundles on every sync
cycle. This caused repo-backups to grow to 17GB+ for users with many
repositories.

Changes:
- Fix resolveBackupStrategy to map backupBeforeSync: true → "on-force-push"
  instead of "always", so legacy configs only backup when force-push is detected
- Fix config mapper to always set backupStrategy explicitly ("on-force-push")
  preventing the backward-compat fallback from triggering
- Lower default backupRetentionCount from 20 to 5 bundles per repo
- Add time-based retention (backupRetentionDays, default 30 days) alongside
  count-based retention, with safety net to always keep at least 1 bundle
- Add "high disk usage" warning on "Always Backup" UI option
- Update docs and tests to reflect new defaults and behavior

* fix: preserve legacy backupBeforeSync:false on UI round-trip and expose retention days

P1: mapDbToUiConfig now checks backupBeforeSync === false before
defaulting backupStrategy, preventing legacy "disabled" configs from
silently becoming "on-force-push" after any auto-save round-trip.

P3: Added "Snapshot retention days" input field to the backup settings
UI, matching the documented setting in FORCE_PUSH_PROTECTION.md.

src/lib/utils/config-defaults.ts

@@ -95,7 +95,8 @@ export async function createDefaultConfig({ userId, envOverrides = {} }: Default
       pullRequestConcurrency: 5,
       backupStrategy: "on-force-push",
       backupBeforeSync: true, // Deprecated: kept for backward compat
-      backupRetentionCount: 20,
+      backupRetentionCount: 5,
+      backupRetentionDays: 30,
       backupDirectory: "data/repo-backups",
       blockSyncOnBackupFailure: true,
     },

src/lib/utils/config-mapper.ts

@@ -101,9 +101,10 @@ export function mapUiToDbConfig(
     mirrorPullRequests: mirrorOptions.mirrorMetadata && mirrorOptions.metadataComponents.pullRequests,
     mirrorLabels: mirrorOptions.mirrorMetadata && mirrorOptions.metadataComponents.labels,
     mirrorMilestones: mirrorOptions.mirrorMetadata && mirrorOptions.metadataComponents.milestones,
-    backupStrategy: giteaConfig.backupStrategy,
+    backupStrategy: giteaConfig.backupStrategy || "on-force-push",
     backupBeforeSync: giteaConfig.backupBeforeSync ?? true,
-    backupRetentionCount: giteaConfig.backupRetentionCount ?? 20,
+    backupRetentionCount: giteaConfig.backupRetentionCount ?? 5,
+    backupRetentionDays: giteaConfig.backupRetentionDays ?? 30,
     backupDirectory: giteaConfig.backupDirectory?.trim() || undefined,
     blockSyncOnBackupFailure: giteaConfig.blockSyncOnBackupFailure ?? true,
   };
@@ -146,9 +147,12 @@ export function mapDbToUiConfig(dbConfig: any): {
     personalReposOrg: undefined, // Not stored in current schema
     issueConcurrency: dbConfig.giteaConfig?.issueConcurrency ?? 3,
     pullRequestConcurrency: dbConfig.giteaConfig?.pullRequestConcurrency ?? 5,
-    backupStrategy: dbConfig.giteaConfig?.backupStrategy || undefined,
+    backupStrategy: dbConfig.giteaConfig?.backupStrategy ||
+      // Respect legacy backupBeforeSync: false → "disabled" mapping on round-trip
+      (dbConfig.giteaConfig?.backupBeforeSync === false ? "disabled" : "on-force-push"),
     backupBeforeSync: dbConfig.giteaConfig?.backupBeforeSync ?? true,
-    backupRetentionCount: dbConfig.giteaConfig?.backupRetentionCount ?? 20,
+    backupRetentionCount: dbConfig.giteaConfig?.backupRetentionCount ?? 5,
+    backupRetentionDays: dbConfig.giteaConfig?.backupRetentionDays ?? 30,
     backupDirectory: dbConfig.giteaConfig?.backupDirectory || "data/repo-backups",
     blockSyncOnBackupFailure: dbConfig.giteaConfig?.blockSyncOnBackupFailure ?? true,
   };