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>

apps/api/openapi.json

@@ -2278,6 +2278,26 @@
               "type": "boolean"
             }
           },
+          {
+            "name": "dueFrom",
+            "required": false,
+            "in": "query",
+            "description": "Due on or after this day",
+            "schema": {
+              "format": "date",
+              "type": "string"
+            }
+          },
+          {
+            "name": "dueTo",
+            "required": false,
+            "in": "query",
+            "description": "Due on or before this day",
+            "schema": {
+              "format": "date",
+              "type": "string"
+            }
+          },
           {
             "name": "sort",
             "required": false,