Added sqlite

lib/services/drive_service.dart

@@ -1,4 +1,3 @@
-import 'dart:convert';
 import 'dart:io';
 
 import 'package:googleapis/drive/v3.dart' as drive;
@@ -6,7 +5,7 @@ import 'package:http/http.dart' as http;
 
 const appFolderName = 'MO-Fuel-Tax-Back';
 const receiptsFolderName = 'receipts';
-const dataFileName = 'fuel_tax_data.json';
+const dataFileName = 'fuel_tax_tracker.db';
 
 const _folderMimeType = 'application/vnd.google-apps.folder';
 
@@ -17,6 +16,16 @@ class DriveFolder {
   DriveFolder({required this.id, required this.name});
 }
 
+class DriveDataFileInfo {
+  final String id;
+
+  /// Cheap change-detection signal: compare against the last value seen to
+  /// decide whether a pull is actually needed, without downloading content.
+  final String? md5Checksum;
+
+  DriveDataFileInfo({required this.id, required this.md5Checksum});
+}
+
 class DriveLockFile {
   final String id;
   final String username;
@@ -87,23 +96,21 @@ class DriveService {
     return created.id!;
   }
 
-  Future<String?> findDataFileId(String appFolderId) async {
+  /// Looks up the shared data file's ID and md5 checksum without
+  /// downloading its content, so sync can cheaply decide whether a pull is
+  /// actually needed.
+  Future<DriveDataFileInfo?> findDataFile(String appFolderId) async {
     final result = await _api.files.list(
       q: "'$appFolderId' in parents and trashed=false and name='$dataFileName'",
       orderBy: 'modifiedTime desc',
-      $fields: 'files(id,name)',
+      $fields: 'files(id,name,md5Checksum)',
       spaces: 'drive',
     );
-    return (result.files ?? []).firstOrNullWithId();
-  }
-
-  Future<String> downloadTextFile(String fileId) async {
-    final media = await _api.files.get(
-      fileId,
-      downloadOptions: drive.DownloadOptions.fullMedia,
-    ) as drive.Media;
-    final bytes = await _collectBytes(media.stream);
-    return utf8.decode(bytes);
+    final files = result.files ?? <drive.File>[];
+    if (files.isEmpty) return null;
+    final first = files.first;
+    if (first.id == null) return null;
+    return DriveDataFileInfo(id: first.id!, md5Checksum: first.md5Checksum);
   }
 
   Future<List<int>> downloadFileBytes(String fileId) async {
@@ -115,17 +122,20 @@ class DriveService {
   }
 
   /// Creates the data file if [existingFileId] is null, otherwise
-  /// overwrites its content. Returns the (possibly new) file ID.
-  Future<String> uploadDataFile({
+  /// overwrites its content with the bytes of the local sqlite database
+  /// file (see [DriveSyncService]). Returns the (possibly new) file ID and
+  /// its fresh md5 checksum, so the caller can remember it for
+  /// change-detection on the next sync without an extra round-trip.
+  Future<DriveDataFileInfo> uploadDataFile({
     required String appFolderId,
     required String? existingFileId,
-    required String jsonContent,
+    required File localSnapshotFile,
   }) async {
-    final bytes = utf8.encode(jsonContent);
+    final length = await localSnapshotFile.length();
     final media = drive.Media(
-      Stream.value(bytes),
-      bytes.length,
-      contentType: 'application/json',
+      localSnapshotFile.openRead(),
+      length,
+      contentType: 'application/x-sqlite3',
     );
 
     if (existingFileId != null) {
@@ -133,8 +143,9 @@ class DriveService {
         drive.File(),
         existingFileId,
         uploadMedia: media,
+        $fields: 'id,md5Checksum',
       );
-      return updated.id ?? existingFileId;
+      return DriveDataFileInfo(id: updated.id ?? existingFileId, md5Checksum: updated.md5Checksum);
     }
 
     final created = await _api.files.create(
@@ -142,8 +153,9 @@ class DriveService {
         ..name = dataFileName
         ..parents = [appFolderId],
       uploadMedia: media,
+      $fields: 'id,md5Checksum',
     );
-    return created.id!;
+    return DriveDataFileInfo(id: created.id!, md5Checksum: created.md5Checksum);
   }
 
   Future<String> uploadReceiptImage({