Chache stored_event_count (on Issue and Projet)

"possibly expensive" turned out to be "actually expensive". On 'emu', with 1.5M
events, the counts take 85 and 154 ms for Project and Issue respectively;
bottlenecking our digestion to ~3 events/s.

Note: this is single-issue, single-project (presumably, the cost would be lower
for more spread-out cases)

Note on indexes: Event already has indexes for both Project & Issue (though as
the first item in a multi-column index). Without checking further: that appears
to not "magically solve counting".

This commit also optimizes the .count() on the issue-detail event list (via
Paginator).

This commit also slightly changes the value passed as `stored_event_count` to
be used for `get_random_irrelevance` to be the post-evication value. That won't
matter much in practice, but is slightly more correct IMHO.

projects/migrations/0010_project_stored_event_count.py

@@ -0,0 +1,18 @@
+# Generated by Django 4.2.18 on 2025-02-06 10:04
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("projects", "0003_project_projects_pr_name_11d782_idx"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="project",
+            name="stored_event_count",
+            field=models.IntegerField(default=0, editable=False),
+        ),
+    ]

projects/migrations/0011_fill_stored_event_count.py

@@ -0,0 +1,23 @@
+from django.db import migrations
+
+def unfill_stored_event_count(apps, schema_editor):
+    Project = apps.get_model("projects", "Project")
+    Project.objects.all().update(stored_event_count=0)
+
+
+def fill_stored_event_count(apps, schema_editor):
+    Project = apps.get_model("projects", "Project")
+    for project in Project.objects.all():
+        project.stored_event_count = project.event_set.count()
+        project.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("projects", "0010_project_stored_event_count"),
+    ]
+
+    operations = [
+        migrations.RunPython(fill_stored_event_count, unfill_stored_event_count),
+    ]

projects/models.py

@@ -52,6 +52,7 @@ class Project(models.Model):
     # denormalized/cached/counted fields below
     has_releases = models.BooleanField(editable=False, default=False)
     digested_event_count = models.PositiveIntegerField(null=False, blank=False, default=0, editable=False)
+    stored_event_count = models.IntegerField(blank=False, null=False, default=0, editable=False)
 
     # alerting conditions
     alert_on_new_issue = models.BooleanField(default=True)