feat: implement database backup and restore functionality with JSON support

Co-authored-by: Copilot <copilot@github.com>

docs/date-range-backup-spec.md

@@ -0,0 +1,81 @@
+# Date-Range Backup Spec
+
+## Status
+
+- Draft only.
+- Do not implement until full-backup import/export has been exercised in real usage.
+
+## Goal
+
+Allow users to export and import only a selected time range without weakening the safety guarantees introduced by full JSON backups.
+
+## Non-goals
+
+- No implementation in this change.
+- No automatic migration across JSON backup format versions.
+- No merging logic for conflicting edits yet.
+
+## Proposed backup modes
+
+- `full`: current behavior, exports every managed collection and can fully replace the database.
+- `dateRange`: future behavior, exports only documents relevant to a user-selected inclusive date interval.
+
+## Proposed JSON shape changes
+
+Add a top-level `scope` object while keeping the existing version markers.
+
+```json
+{
+  "backupFormatVersion": 1,
+  "databaseSchemaVersion": 1,
+  "exportedAtUtc": "2026-04-24T12:00:00Z",
+  "scope": {
+    "mode": "full"
+  },
+  "collections": {
+    "app_settings": [],
+    "users": [],
+    "workdays": []
+  }
+}
+```
+
+Future date-range backups would use:
+
+```json
+"scope": {
+  "mode": "dateRange",
+  "from": "2026-04-01",
+  "to": "2026-04-30"
+}
+```
+
+## Selection rules for `dateRange`
+
+- `workdays`: include documents whose logical day falls within `from` and `to` inclusive.
+- `app_settings`: exclude by default. Settings are global, not time-scoped.
+- `users`: exclude by default. Authentication data is global, not time-scoped.
+- Future collections must declare whether they are `global`, `timeScoped`, or `derived` before they participate in range backups.
+
+## Import semantics for `dateRange`
+
+- Import must not reuse the current full-overwrite flow.
+- Import must require a different confirmation message than full restore.
+- Import must define conflict rules before implementation.
+- Preferred first version: replace only the selected date range inside time-scoped collections.
+- Global collections must remain untouched unless explicitly opted in by the user and supported by the backup format version.
+
+## Compatibility and versioning requirements
+
+- Keep `backupFormatVersion` and `databaseSchemaVersion` mandatory for every backup mode.
+- If `scope.mode` is unknown, reject import.
+- If `backupFormatVersion` is unsupported, reject import and surface that a migration/importer is required.
+- If `databaseSchemaVersion` is unsupported, reject import and surface that a schema migration is required.
+- When date-range import is implemented, version-specific import handlers should be introduced behind a dispatch layer such as `ImportV1Full`, `ImportV1DateRange`, and future `ImportV2*` handlers.
+
+## UI requirements for future work
+
+- Add date pickers for `from` and `to`.
+- Validate `from <= to` before export/import is enabled.
+- Show a summary of what will be affected before confirming import.
+- Keep full-backup export/import available as the safer recovery path.