Count only active learners against the learners cap

[payamnj]

Summary

`Organization.can_enroll_learner()` (`django_email_learning/models/organizations.py`) previously counted every `Learner` row belonging to the organization against `MAX_LEARNERS_PER_ORGANIZATION`, regardless of enrollment status. A learner with no enrollment at all, or only an unverified/completed/deactivated one, still counted toward the cap.

It now counts distinct learners with at least one `EnrollmentStatus.ACTIVE` enrollment:

```python
def can_enroll_learner(self) -> bool:
cap = self.get_learners_cap()
if not cap:
return True
active_learner_count = self.learner_set.filter(enrollments__status=EnrollmentStatus.ACTIVE).distinct().count()
return active_learner_count < cap

\`.distinct()\` matters here — without it, a single learner active in multiple courses would be counted once per enrollment (multiple joined rows), inflating the count.

## Test changes

- Every existing cap test across the enrollment paths (platform \`EnrollmentsView\`, public \`EnrollView\`, \`EnrollCommand\`, the organization API's \`can_enroll_learner\` field, and the public \`enrollmentOpen\` context) created a bare \`Learner\` with no enrollment to simulate "cap reached." Updated each to give that learner an active enrollment, since a bare \`Learner\` no longer counts.
- Added \`test_can_enroll_learner_ignores_learners_without_active_enrollment\` — learners with no enrollment, or only unverified/completed/deactivated ones, don't count.
- Added \`test_can_enroll_learner_counts_distinct_learners_not_enrollments\` — a learner active in two courses only counts once. I verified this test actually catches a missing \`.distinct()\` (temporarily removed it, confirmed the test fails, restored it) before finalizing.

## Docs

Updated \`docs/source/platform/learners.rst\`'s "Learner Capacity" section to document the active-only counting.

## Test plan

- [x] \`pytest -q\` — 786 passed, 81.84% coverage
- [x] \`mypy\` — no issues across 164 source files
- [x] \`python manage.py makemigrations --check\` — no drift (query-only change, no schema change)
- [x] \`ruff check\` / \`ruff format\` — clean
- [x] Manually confirmed the distinct-count test fails without \`.distinct()\` and passes with it

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