feat(eap): add downsampled event retention to config (#96818)

depends on getsentry/relay#5013

This PR is the second change needed to resolve
https://linear.app/getsentry/issue/EAP-160/modify-the-org-setting-to-reflect-the-retention-change-based-on-the.

Author: brendanhsentry

src/sentry/quotas/base.py

@@ -401,6 +401,13 @@ def get_event_retention(self, organization):
         """
         return _limit_from_settings(options.get("system.event-retention-days"))
 
+    def get_downsampled_event_retention(self, organization):
+        """
+        Returns the retention for downsampled events in the given organization in days.
+        Returning ``0`` means downsampled event retention will default to the value of ``get_event_retention``.
+        """
+        return 0
+
     def validate(self):
         """
         Validates that the quota service is operational.

src/sentry/relay/config/__init__.py

@@ -1150,6 +1150,12 @@ def _get_project_config(
         event_retention = quotas.backend.get_event_retention(project.organization)
         if event_retention is not None:
             config["eventRetention"] = event_retention
+    with sentry_sdk.start_span(op="get_downsampled_event_retention"):
+        downsampled_event_retention = quotas.backend.get_downsampled_event_retention(
+            project.organization
+        )
+        if downsampled_event_retention is not None:
+            config["downsampledEventRetention"] = downsampled_event_retention
     with sentry_sdk.start_span(op="get_all_quotas"):
         if quotas_config := get_quotas(project, keys=project_keys):
             config["quotas"] = quotas_config

tests/sentry/api/endpoints/test_relay_projectconfigs.py

@@ -150,6 +150,9 @@ def test_internal_relays_should_receive_full_configs(
     assert safe.get_path(cfg, "config", "eventRetention") == quotas.backend.get_event_retention(
         default_project.organization
     )
+    assert safe.get_path(
+        cfg, "config", "downsampledEventRetention"
+    ) == quotas.backend.get_downsampled_event_retention(default_project.organization)
 
 
 @django_db_all

tests/sentry/api/endpoints/test_relay_projectconfigs_v2.py

@@ -106,6 +106,9 @@ def test_internal_relays_should_receive_full_configs(
     assert safe.get_path(cfg, "config", "eventRetention") == quotas.backend.get_event_retention(
         default_project.organization
     )
+    assert safe.get_path(
+        cfg, "config", "downsampledEventRetention"
+    ) == quotas.backend.get_downsampled_event_retention(default_project.organization)
 
 
 @django_db_all

tests/sentry/relay/snapshots/test_config/test_get_project_config/REGION.pysnap

@@ -1,5 +1,5 @@
 ---
-created: '2025-08-06T00:04:51.292281+00:00'
+created: '2025-08-11T23:31:41.914727+00:00'
 creator: sentry
 source: tests/sentry/relay/test_config.py
 ---
@@ -20,6 +20,7 @@ config:
     scrubData: true
     scrubDefaults: true
     sensitiveFields: []
+  downsampledEventRetention: 0
   filterSettings:
     csp:
       disallowedSources: