Initial commit: CookIT recipe collection and week planner

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/src/planning/models/plan_entry.dart

@@ -0,0 +1,22 @@
+/// One thing planned for a Meal on a day: a Recipe or a short note.
+class PlanEntry {
+  const PlanEntry({
+    required this.id,
+    required this.day,
+    required this.mealId,
+    this.recipeId,
+    this.note,
+    required this.position,
+  }) : assert((recipeId == null) != (note == null), 'A Plan Entry is either a Recipe or a note');
+
+  final String id;
+
+  /// The calendar day, as a UTC midnight (see `day.dart`).
+  final DateTime day;
+  final String mealId;
+  final String? recipeId;
+  final String? note;
+
+  /// Order within its Meal on its day.
+  final int position;
+}