feat: scope immediate notification email batching per user (#38866)

* feat: scope immediate notification email batching per user

Immediate-email batching was scoped by user AND course_id, so a learner
enrolled in multiple active courses received a separate buffered digest
per course. Notification preferences are now account-level, so batching
should feel account-level from the learner's perspective too.

Scope the buffer by user only in decide_email_action,
schedule_digest_buffer and send_buffered_digest: the first eligible
notification is sent immediately, and additional immediate notifications
within NOTIFICATION_IMMEDIATE_EMAIL_BUFFER_MINUTES are grouped into a
single digest regardless of which course generated them. The digest
already resolves per-notification course names, so multi-course digests
render unchanged.

course_key is retained in send_buffered_digest's signature so digest
tasks enqueued before the change still deserialize.

Closes openedx/tutor-contrib-platform-notifications#66

* fix: anchor buffered digest to last immediate email so wait stays buffer_minutes

The digest was scheduled buffer_minutes from when the SECOND notification was
processed, so the total wait after the immediate email was (gap to 2nd
notification) + buffer_minutes -- observed as ~22 min for a 15 min buffer.

Anchor scheduled_time to last_sent.email_sent_on + buffer_minutes instead, and
clamp to now() when that window has already elapsed. Add tests for both the
relative-to-last-email schedule and the already-elapsed fast path.