fix(instructor): bulk-enroll auto-async, chunking, duplicate guards

[AliAlfaifi]

Why

Bulk enrollment from the instructor dashboard (Membership → Batch Enrollment) fails in prod in three measured ways (2026-09-06→10-06):

1. Sync + notify costs ~2 s/learner → ~60 learners exceed Cloudflare's 125 s → browser error → re-click → duplicate runs; learners mailed up to 8×.
2. Async refuses JSON task_input > 10,000 chars (~330–380 emails) with an uncaught 500 "Task was not created".
3. No guard against the same batch being submitted repeatedly.

What (each divergence marked # NELC: for the Dec-2026 re-port)

• instructor_task/models.py: TASK_INPUT_LENGTH 10,000 → 60,000 (task_input is a TEXT column, no migration). Over-limit raises TaskInputTooLongError (subclass of AttributeError, so existing callers keep working).
• instructor_task/api.py: split_enrollment_identifiers chunks a batch so each task fits; raises instead of truncating.
• instructor/views/api.py (StudentsUpdateEnrollmentView, dashboard POST only):
  • auto-switch to async above BATCH_ENROLLMENT_SYNC_MAX_NOTIFY (30, notify on) / BATCH_ENROLLMENT_SYNC_MAX (100, notify off); response carries auto_switched_to_async, sync_limit;
  • batches too big for one task become several tasks (task_ids);
  • unfittable input → 400 with a clear message (was 500);
  • sync path takes a cache.add lock on (course, action, md5 of sorted identifiers) → 409 if already running, released in finally (BATCH_ENROLLMENT_LOCK_TIMEOUT_SECONDS=900).
• instructor/utils.py: skip re-sending the notify email when the learner's latest audit row for this course already reached the requested state within BATCH_ENROLLMENT_EMAIL_DEDUPE_SECONDS (900); result row gets email_skipped: true.
• membership.js: buttons disabled while a request is pending; server 400/409 messages shown; auto-switch/split explained to the instructor.
• /bulk_enroll REST API unchanged (guarded by a guard_ui_request flag) so its response contract does not break.

Testing

Container (overhangio/openedx:20.0.5 + this fork, sqlite + mongo): baseline 348 passed → 378 passed in the touched modules (30 new tests); 856 passed across instructor, instructor_task, bulk_enroll. pylint/pycodestyle clean on changed files; node --check on the JS.

Known limits / to verify on stage

• The lock needs a cache shared across LMS pods (Tutor uses Redis; not confirmed on the pods).
• Email dedupe does not cover invited-but-unregistered learners (no audit→enrollment link).
• One chunk can hold ~1,700 identifiers (~1 h with notify on until the pooled SMTP backend lands).
• JS behaviour not exercised in a browser; ESLint not run.

Related: nelc/eox-nelp feat/pooled-smtp-email-backend (email speed), nelc/nassau feat/pooled-smtp-email-backend-stage.

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