feat(server): partage ciblé de programmes et séances entre comptes (ticket #51)

Ajoute l'endpoint api/share_api.dart, les use cases de partage
(application/share_use_cases.dart) et l'adapter Postgres
(infrastructure/postgres/share_repository.dart). L'acceptation d'un
partage crée une nouvelle ressource synced_resources avec de nouveaux
IDs pour le destinataire, sans jamais modifier la ressource source de
l'émetteur ; gestion des conflits révoqué/déjà répondu. dart pub get
OK, dart analyze clean, dart test 24/24 vert. Use case d'acceptation
relu manuellement et jugé correct ; pas de test de bout en bout contre
un vrai PostgreSQL faute d'accès Docker dans ce sandbox.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/lib/domain/README.md

@@ -5,4 +5,5 @@ Pure server domain entities and invariants.
 This layer must not import Shelf, PostgreSQL adapters, Docker configuration or
 other infrastructure concerns.
 
-Current entities: `UserAccount`, `AuthSession` and `SyncedResource`.
+Current entities: `UserAccount`, `AuthSession`, `SyncedResource`, `Share` and
+`ShareRecipient`.

server/lib/domain/entities.dart

@@ -23,6 +23,14 @@ final class UnauthorizedException extends DomainException {
   const UnauthorizedException() : super('Unauthorized.');
 }
 
+final class ShareNotFoundException extends DomainException {
+  const ShareNotFoundException() : super('Share not found.');
+}
+
+final class ShareConflictException extends DomainException {
+  const ShareConflictException(super.message);
+}
+
 final class UserAccount {
   UserAccount({
     required String id,
@@ -145,6 +153,76 @@ final class SyncedResource {
   final String? originDeviceId;
 }
 
+enum ShareRecipientStatus {
+  pending('pending'),
+  accepted('accepted'),
+  declined('declined'),
+  revoked('revoked');
+
+  const ShareRecipientStatus(this.wireName);
+
+  final String wireName;
+
+  static ShareRecipientStatus parse(String value) {
+    for (final status in values) {
+      if (status.wireName == value) {
+        return status;
+      }
+    }
+    throw ValidationException('Unsupported share recipient status: $value.');
+  }
+}
+
+final class Share {
+  Share({
+    required String id,
+    required String senderUserId,
+    required this.resourceType,
+    required Map<String, Object?> payloadJson,
+    required DateTime createdAt,
+    DateTime? revokedAt,
+  }) : id = _nonBlank(id, 'Share id'),
+       senderUserId = _nonBlank(senderUserId, 'Sender user id'),
+       payloadJson = Map.unmodifiable(payloadJson),
+       createdAt = createdAt.toUtc(),
+       revokedAt = revokedAt?.toUtc() {
+    if (resourceType != SyncedResourceType.program &&
+        resourceType != SyncedResourceType.workoutTemplate) {
+      throw const ValidationException(
+        'Shares only support program and workoutTemplate resources.',
+      );
+    }
+  }
+
+  final String id;
+  final String senderUserId;
+  final SyncedResourceType resourceType;
+  final Map<String, Object?> payloadJson;
+  final DateTime createdAt;
+  final DateTime? revokedAt;
+
+  bool get isRevoked => revokedAt != null;
+}
+
+final class ShareRecipient {
+  ShareRecipient({
+    required String id,
+    required String shareId,
+    required String recipientUserId,
+    this.status = ShareRecipientStatus.pending,
+    DateTime? respondedAt,
+  }) : id = _nonBlank(id, 'Share recipient id'),
+       shareId = _nonBlank(shareId, 'Share id'),
+       recipientUserId = _nonBlank(recipientUserId, 'Recipient user id'),
+       respondedAt = respondedAt?.toUtc();
+
+  final String id;
+  final String shareId;
+  final String recipientUserId;
+  final ShareRecipientStatus status;
+  final DateTime? respondedAt;
+}
+
 String _nonBlank(String value, String label) {
   final trimmed = value.trim();
   if (trimmed.isEmpty) {