Add email as a recognized, feature-flagged notification service type.
The flag defaults to false and acts as a dispatch gate alongside the
existing discord, gotify, and webhook notification service flags.
- Add FlagEmailServiceEnabled constant to the notifications feature flag
registry with the canonical key convention
- Register the flag in the handler defaults so it appears in the feature
flags API response with a false default
- Recognise 'email' as a supported notification provider type so that
providers of this type pass the type validation gate
- Gate email dispatch on the new flag in isDispatchEnabled() following
the same pattern as gotify and webhook service flags
- Expand the E2E test fixtures FeatureFlags interface to include the new
flag key so typed fixture objects remain accurate
No email message dispatch is wired in this commit; the flag registration
alone makes the email provider type valid and toggleable.
5a58404e1b