feat(ideas): add idea creator, grant calls and applications

Implement the idea creator and application generator: idea status workflow, social canvas card, grant call windows with 409 when closed, and an LLM-backed idea assistant with offline fallback.

backend/Domain/Rules/ApplicationWindow.cs

@@ -0,0 +1,13 @@
+namespace TarninaHub.Domain.Rules;
+
+public static class ApplicationWindow
+{
+    public const string Open = "open";
+
+    /// <summary>
+    /// Applications are only allowed while the call is open and the current
+    /// moment is within [opensAt, closesAt].
+    /// </summary>
+    public static bool IsOpen(string status, DateTimeOffset opensAt, DateTimeOffset closesAt, DateTimeOffset now) =>
+        string.Equals(status, Open, StringComparison.Ordinal) && now >= opensAt && now <= closesAt;
+}