Add program times for items (Z#23178639)

* Add program times for items

* Fix frontend date validation

* Add ical data for program times [wip]

* Improve ical data for program times

* Remove duplicate code and add comments

* Adjust migration

* Remove program times form for event series

* Add pdf placeholder [wip]

* Improve explanation text with suggestion

Co-authored-by: Raphael Michel <michel@pretix.eu>

* Fix import sorting

* Improve ical generation

* Improve ical entry description

* Fix migration

* Add copyability for program times fot items and events

* Update migration

* Add API endpoints/functions, fix isort

* Improve variable name

Co-authored-by: Richard Schreiber <schreiber@rami.io>

* Remove todo comment

* Add documentation, Change endpoint name

* Change related name

* Remove unnecessary code block

* Add program times to item API

* Fix imports

* Add log text

* Use daterange helper

* Add and update API tests

* Add another API test

* Add program times to cloning tests

* Update query count because of program times query

* Invalidate cached tickets on program time changes

* Reduce invalidation calls

* Update migration after rebase

* Apply improvements to invalidation from review

Co-authored-by: Richard Schreiber <schreiber@rami.io>

* remove unneccessary attr=item param

* remove unnecessary kwargs for formset_factory

* fix local var name being overwritten in for-loop

* fix empty formset being saved

* Use subevent if available

* make code less verbose

* remove double event-label in ical desc

* fix unnecessary var re-assign

* fix ev vs p.subevent

---------

Co-authored-by: Raphael Michel <michel@pretix.eu>
Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/pretix/base/migrations/0294_item_program_time.py

@@ -0,0 +1,25 @@
+# Generated by Django 4.2.19 on 2025-08-11 10:25
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0293_cartposition_price_includes_rounding_correction_and_more'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='ItemProgramTime',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False)),
+                ('start', models.DateTimeField()),
+                ('end', models.DateTimeField()),
+                ('item',
+                 models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='program_times',
+                                   to='pretixbase.item')),
+            ],
+        ),
+    ]