support env var substitution for notification email (#24517)

`notifications.email` is now an `EnvString`, so it can come from `secrets.yaml`, a Docker secret, or a container env var. `/api/config` returns the resolved value, so the email is now redacted for non-admin users, including each camera's inherited copy and the profile `base_config` copy.

frigate/test/http_api/test_http_app.py

@@ -4,6 +4,7 @@ from unittest.mock import Mock, patch
 
 import frigate.genai
 from frigate.config import GenAIProviderEnum
+from frigate.config.env import FRIGATE_ENV_VARS
 from frigate.const import MODEL_CACHE_DIR, REDACTED_CREDENTIAL_SENTINEL
 from frigate.genai import GenAIClient
 from frigate.models import Event, Recordings, ReviewSegment
@@ -111,6 +112,30 @@ class TestHttpApp(BaseTestHttp):
             mqtt = response.json()["mqtt"]
             assert mqtt["password"] == REDACTED_CREDENTIAL_SENTINEL
 
+    def test_config_response_hides_notification_email_from_viewers(self):
+        self.minimal_config["notifications"] = {"email": "{FRIGATE_TEST_EMAIL}"}
+
+        with patch.dict(FRIGATE_ENV_VARS, {"FRIGATE_TEST_EMAIL": "me@example.com"}):
+            app = super().create_app()
+
+        assert app.frigate_config.notifications.email == "me@example.com"
+
+        with AuthTestClient(app) as client:
+            response = client.get(
+                "/config",
+                headers={"remote-user": "viewer", "remote-role": "viewer"},
+            )
+            assert response.status_code == 200
+            config = response.json()
+            assert config["notifications"]["email"] == REDACTED_CREDENTIAL_SENTINEL
+            assert (
+                config["cameras"]["front_door"]["notifications"]["email"]
+                == REDACTED_CREDENTIAL_SENTINEL
+            )
+
+            response = client.get("/config")
+            assert response.json()["notifications"]["email"] == "me@example.com"
+
     def test_config_response_keeps_plus_model_reference(self):
         model_id = "test_plus_reference"
         model_path = os.path.join(MODEL_CACHE_DIR, model_id)