feat(meditate): the operator's library on the server, and a course's lessons done synced

An operator's folder (MEDITATE_LIBRARY_DIR) of recordings, pictures and a
YAML manifest, library.yaml (commented sample: library.example.yaml), read
as the server starts and every ten minutes, held in memory. Items without
a licence or credit, files outside the folder, unknown categories and
courses pointing at items not served are refused and logged; a manifest
that doesn't read keeps the library served before. bin/check_library.dart
checks a folder before deploying.

The catalogue goes to signed-in people alone (LibraryEndpoint), with a
12-hour access the server signs; the web server's /library/<served name>
serves the files to that access only, by ranges (relic's StaticHandler).
No request path reaches the file system.

lesson_done: a lesson done, sealed and synced, a row each; the migration
creates the table with its spaceId index and the cascading foreign key to
the sync engine's spaces, as dream's did. ADR 0038, D16.

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/meditate/halkyon_meditate_server/pubspec.yaml

@@ -9,6 +9,9 @@ resolution: workspace
 
 dependencies:
   clock: ^1.1.3
+  # The library's served names and the signature of its accesses (ADR 0038,
+  # D16).
+  crypto: ^3.0.7
   # What every Halkyon server has (ADR 0014): signing in, the account and its
   # deletion, pruning, the instance's basics. A Serverpod module among the
   # repository's shared packages.
@@ -17,6 +20,8 @@ dependencies:
   serverpod: 4.0.4
   serverpod_auth_idp_server: 4.0.4
   serverpod_offline_sync_server: 0.0.10
+  # The operator's library manifest, `library.yaml` (ADR 0038, D16).
+  yaml: ^3.1.4
 
 dev_dependencies:
   serverpod_test: 4.0.4