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.

docs/FORCE_PUSH_PROTECTION.md

@@ -78,7 +78,11 @@ These appear when any non-disabled strategy is selected:
 
 ### Snapshot Retention Count
 
-How many backup snapshots to keep per repository. Oldest snapshots are deleted when this limit is exceeded. Default: **20**.
+How many backup snapshots to keep per repository. Oldest snapshots are deleted when this limit is exceeded. Default: **5**.
+
+### Snapshot Retention Days
+
+Maximum age (in days) for backup snapshots. Bundles older than this are deleted during retention enforcement, though at least one bundle is always kept. Set to `0` to disable time-based retention. Default: **30**.
 
 ### Snapshot Directory
 
@@ -96,7 +100,7 @@ The old `backupBeforeSync` boolean is still recognized:
 
 | Old Setting | New Equivalent |
 |---|---|
-| `backupBeforeSync: true` | `backupStrategy: "always"` |
+| `backupBeforeSync: true` | `backupStrategy: "on-force-push"` |
 | `backupBeforeSync: false` | `backupStrategy: "disabled"` |
 | Neither set | `backupStrategy: "on-force-push"` (new default) |