refactor(app,learn): people's files on the device shared in halkyon_app

ADR 0043's recommendation, for Todo's attachments: what Learn had for its
files on the device moves into halkyon_app, unchanged but for its names,
and Learn uses it from there.

- FileCache, DirectoryFileCache (Learn's AssetFileCache): the files by
  their SHA-256 in a folder of the person's, and which wait to be
  uploaded.
- FileServerGateway, ServerpodFileServerGateway (Learn's
  AssetServerGateway): halkyon_base's files endpoint, taking the module's
  caller rather than Learn's client; FileLimitReached, FileRoom.
- Their fakes in halkyon_app's testing: MemoryFileCache,
  FakeFileServerGateway, sha256Of.
- SealedFile: the format every app seals a file in (HLKF, version 1),
  sealing with halkyon_app's ContentCipher; Learn's Privacy seals and
  opens its files with it, for the same context as before, so every file
  sealed so far opens as it did.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/test/sync/support/sync_devices.dart

@@ -8,9 +8,7 @@ import 'package:crypto/crypto.dart';
 import 'package:halkyon_app/halkyon_app.dart';
 import 'package:halkyon_app/testing.dart';
 import 'package:halkyon_learn_client/halkyon_learn_client.dart' as protocol;
-import 'package:halkyon_learn_flutter/src/core/asset/data/asset_file_cache.dart';
 import 'package:halkyon_learn_flutter/src/core/asset/data/asset_repository.dart';
-import 'package:halkyon_learn_flutter/src/core/asset/data/serverpod_asset_server_gateway.dart';
 import 'package:halkyon_learn_flutter/src/core/asset/data/sync_asset_gateway.dart';
 import 'package:halkyon_learn_flutter/src/core/privacy/data/content_keys.dart';
 import 'package:halkyon_learn_flutter/src/core/privacy/data/privacy.dart';
@@ -278,8 +276,8 @@ final class SyncDevice(
     );
     final assets = AssetRepository(
       SyncAssetGateway(db, syncRequests, privacy),
-      ServerpodAssetServerGateway(client),
-      DirectoryAssetFileCache(Directory('${database.path}.files')),
+      ServerpodFileServerGateway(client.modules.base),
+      DirectoryFileCache(Directory('${database.path}.files')),
       privacy,
     );
     return SyncDevice(