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/config/camera/notification.py

@@ -1,6 +1,7 @@
 from pydantic import Field
 
 from ..base import FrigateBaseModel
+from ..env import EnvString
 
 __all__ = ["NotificationConfig"]
 
@@ -11,7 +12,7 @@ class NotificationConfig(FrigateBaseModel):
         title="Enable notifications",
         description="Enable or disable notifications for all cameras; can be overridden per-camera.",
     )
-    email: str | None = Field(
+    email: EnvString | None = Field(
         default=None,
         title="Notification email",
         description="Email address used for push notifications or required by certain notification providers.",