API: project roster titles track the latest position overlapping the role (#1435)

[jonfroehlich]

Closes #1435 — though not the way the issue proposed. Short version: the issue asked to add current_title / current_school to the nested person; that treats a symptom. The roster's own position_* fields were stale, and fixing them there is both correct and enough.

The defect

position_title / position_school / position_school_abbreviated on /api/v1/projects/<slug>/people/ are pinned to the Position held on the role's start date (the rule set in #1426). A role that's been open since 2012 therefore reports a 2012 title. On prod right now:

endpoint	field	value
/api/v1/people/jonfroehlich/	current_title / current_school	Professor / University of Washington
/api/v1/projects/sidewalk/people/	position_title / position_school_abbreviated	Assistant Professor / UMD (role 2012-02-01 → open)

That gap is exactly why Sidewalk's About page issues one /people/<url_name>/ request per active member — 11 today — to fill in two strings.

The rule

ProjectRole.position_during_role now resolves the latest Position overlapping the role's window, [start_date, end_date or today]:

• Ongoing role → what the person is today. Jon's Sidewalk row becomes Professor / University of Washington.
• Finished stint → still frozen in its own time. A 2015 undergrad who is a professor now still reads Undergrad next to that stint, because the professorship never overlapped it. If their title changed mid-stint (undergrad → MS), the later one wins.

Fallbacks are unchanged in spirit, re-anchored from the role's start to the window's end: role in a gap between positions → most recent already begun; role predating every position → the earliest; no positions → null.

Two guards: the window end is clamped to at least start_date (a typo'd end_date degrades to "position at role start" instead of inverting the range and matching nothing), and a Position starting in the future — a promotion entered ahead of time — is excluded, since the window stops at today.

Why not what the issue asked for

current_title is project-agnostic and, for an alum, describes their last lab position rather than today's employer — so it's wrong for a past-contributor card in a different way. It also lives on PersonSummarySerializer, which is nested in every publication's authors; that endpoint prefetches authors but not authors__position_set, so it would go N+1 on a list of pubs for a field nobody there reads.

Consumer note

⚠️ position_title / position_school values change for roles that span a title change — that's the fix, not a side effect. Field names and types are untouched, so test/js/aboutPage.test.js shape assertions hold; ProjectSidewalk/SidewalkWebpage#4664 can now drop its per-member Promise.allSettled block and read the two fields straight off the roster. Documented in docs/API.md as the second deliberate v1 exception, alongside #1432.

Testing

• website/tests/test_project_role.py rewritten around the new rule (model level, where a failure isolates): ongoing-role-reports-today, mid-stint promotion, ended-role-ignores-later-positions, future-position-excluded, inverted-window clamp, plus the unchanged gap / predates-all / no-positions / pk-tie-break / assertNumQueries(0) prefetch cases.
• website/tests/test_api.py pins the wire contract: the multi-position record now serializes MS Student / UW, and a new test asserts an active role serializes today's title. The N+1 guard is unchanged and still flat.
• Full suite: 712 tests, OK (python manage.py test website --settings=makeabilitylab.settings_test).
• Spot-checked against real data on the local stack: Jon's Sidewalk row now reads Professor / University of Washington.

No UI change, so no Pa11y run. Version bumped to 2.31.0.

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