feat: complete e621 integration with API key auth, comics/pools, post variations, and server sync (v3.7.0)

lib/backend/models/e621_pool.dart

@@ -0,0 +1,58 @@
+class E621Pool {
+  const E621Pool({
+    required this.id,
+    required this.name,
+    required this.description,
+    required this.postIds,
+    required this.postCount,
+    required this.category,
+    required this.isActive,
+    this.creatorName,
+  });
+
+  final int id;
+  final String name;
+  final String description;
+  final List<String> postIds;
+  final int postCount;
+  final String category; // 'series' or 'collection'
+  final bool isActive;
+  final String? creatorName;
+
+  String get cleanTitle => name.replaceAll('_', ' ');
+
+  int pageOf(String postId) {
+    final idx = postIds.indexOf(postId);
+    return idx >= 0 ? idx + 1 : 0;
+  }
+
+  String? nextPostId(String postId) {
+    final idx = postIds.indexOf(postId);
+    if (idx >= 0 && idx < postIds.length - 1) {
+      return postIds[idx + 1];
+    }
+    return null;
+  }
+
+  String? previousPostId(String postId) {
+    final idx = postIds.indexOf(postId);
+    if (idx > 0) {
+      return postIds[idx - 1];
+    }
+    return null;
+  }
+
+  factory E621Pool.fromJson(Map<String, dynamic> json) {
+    final rawPostIds = (json['post_ids'] as List?) ?? const [];
+    return E621Pool(
+      id: (json['id'] as num?)?.toInt() ?? 0,
+      name: (json['name'] ?? '').toString(),
+      description: (json['description'] ?? '').toString(),
+      postIds: rawPostIds.map((e) => e.toString()).toList(),
+      postCount: (json['post_count'] as num?)?.toInt() ?? rawPostIds.length,
+      category: (json['category'] ?? 'series').toString(),
+      isActive: (json['is_active'] as bool?) ?? true,
+      creatorName: json['creator_name']?.toString(),
+    );
+  }
+}