Performance: Add DB indexes for common query/sort patterns (#13167)

Adds indexes on Document.checksum, Document.page_count, and a composite
(owner, created) index on Document, plus composite (field, value_*)
indexes on CustomFieldInstance for each typed value column.

Benchmarked against a 100k-document / 200k-custom-field-instance SQLite
dataset: Document.checksum lookups ~69x faster, page_count sort ~40x,
owner+created list queries ~6x. CustomFieldInstance composite indexes
show large gains (10-24x) on realistic narrow-selectivity queries
matching the workflow scheduler and custom-field search code paths.

src/documents/migrations/0022_add_perf_indexes.py

@@ -0,0 +1,73 @@
+# Generated by Django 5.2.16 on 2026-07-19 20:33
+
+import django.core.validators
+from django.conf import settings
+from django.db import migrations
+from django.db import models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("documents", "0021_widen_workflow_integer_fields"),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="document",
+            name="checksum",
+            field=models.CharField(
+                db_index=True,
+                editable=False,
+                help_text="The checksum of the original document.",
+                max_length=64,
+                verbose_name="checksum",
+            ),
+        ),
+        migrations.AlterField(
+            model_name="document",
+            name="page_count",
+            field=models.PositiveIntegerField(
+                db_index=True,
+                help_text="The number of pages of the document.",
+                null=True,
+                validators=[django.core.validators.MinValueValidator(1)],
+                verbose_name="page count",
+            ),
+        ),
+        migrations.AddIndex(
+            model_name="customfieldinstance",
+            index=models.Index(
+                fields=["field", "value_date"],
+                name="documents_c_field_i_30b51d_idx",
+            ),
+        ),
+        migrations.AddIndex(
+            model_name="customfieldinstance",
+            index=models.Index(
+                fields=["field", "value_int"],
+                name="documents_c_field_i_d25ab0_idx",
+            ),
+        ),
+        migrations.AddIndex(
+            model_name="customfieldinstance",
+            index=models.Index(
+                fields=["field", "value_float"],
+                name="documents_c_field_i_c35174_idx",
+            ),
+        ),
+        migrations.AddIndex(
+            model_name="customfieldinstance",
+            index=models.Index(
+                fields=["field", "value_monetary_amount"],
+                name="documents_c_field_i_53ce4e_idx",
+            ),
+        ),
+        migrations.AddIndex(
+            model_name="document",
+            index=models.Index(
+                fields=["owner", "created"],
+                name="documents_d_owner_i_ec6ab3_idx",
+            ),
+        ),
+    ]