Add data export functionality with gzip compression

- Add FindByUserID method to HabitEntryRepository (JOIN with habits)
- Implement ExportUserDataHandler to export all user data
- Add GET /api/v1/export endpoint with gzip compression
- Apply strict rate limiting (1 export per hour per user)
- Export includes all habits (active + archived) and entries
- Add i18n translations for export errors (en/es)
- Update all test mocks to implement new repository method
- Export format: JSON with gzip (~10x compression ratio)

internal/i18n/locales/en.json

@@ -47,6 +47,7 @@
     "failed_reset_password": "Failed to reset password",
     "failed_delete_user": "Failed to delete user",
     "failed_get_stats": "Failed to get statistics",
+    "export_failed": "Failed to export data",
     "timezone_required": "Timezone is required",
     "invalid_timezone": "Invalid timezone (must be a valid IANA timezone)"
   },