feat: a sprint calendar

A Calendario tab lays the project's sprints out month by month as bands,
with the releases and the tickets due on each day. Owners and admins plan
a sprint by clicking the day it starts, and the whole plan downloads as an
iCalendar file. Tickets can be searched by due-date range.

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

libs/api/tracker/infrastructure/src/lib/mongo/find.repositories.ts

@@ -413,6 +413,13 @@ function toMongoFilter(filter: IssueFilter): FilterQuery<IssueDoc> {
     query.labels = { $all: filter.labels.map((l) => l.toLowerCase()) };
   if (filter.releaseId) query['release.id'] = filter.releaseId;
   if (filter.parentId) query['parent.id'] = filter.parentId;
+  if (filter.dueFrom || filter.dueTo)
+    query['dueDate'] = {
+      $ne: null,
+      ...(filter.dueFrom ? { $gte: filter.dueFrom } : {}),
+      // Full ISO dates on the last day still count.
+      ...(filter.dueTo ? { $lte: `${filter.dueTo}\uffff` } : {}),
+    };
   if (filter.noSprint) query['sprint'] = null;
   else if (filter.sprintId) query['sprint.id'] = filter.sprintId;
   if (filter.unassigned) query['assignee'] = null;