FAC-146 fix: order semesters by academic start_date

[y4nder]

Summary

Semester.createdAt reflects DB insertion time, not academic chronology. The moment a past semester is backfilled (e.g. S12526) after the current one (S22526), every query that used ORDER BY created_at to find "latest" or "previous" semester silently returns the wrong row.

The worst offender is mv_faculty_trends.ordinal, whose regr_slope(score, ordinal) drives the improving/declining classification on the trends page — with inverted ordering, an improving faculty reads as declining and vice versa.

Scope

Schema

• Add startDate (NOT NULL, indexed) + endDate (nullable) to Semester.
• Backfill existing rows by parsing code (S22526 → Jan 20 – Jun 1 2026) with the same calendar as admin.faculytics getSemesterDates():
  • Sem 1: Aug 1 – Dec 18 of startYear
  • Sem 2: Jan 20 – Jun 1 of endYear
  • Sem 3 (intersession): Jun 15 – Jul 31 of endYear
• Rebuild mv_faculty_trends with ORDER BY s.start_date for the per-faculty ordinal window.

Sync

• parseSemesterCode() returns startDate/endDate; processSemesters() upserts them so re-syncs correct backfilled values.

Query sites swapped from Semester.created_at to Semester.start_date

• analytics.service.ts — GetDepartmentOverview previous-semester lookup
• analytics.service.ts — GetFacultyTrends latest-semester fallback
• semesters.service.ts — GET /semesters list ordering (so the frontend switcher defaults to the correct "latest" term)
• mv_faculty_trends.ordinal window

DTO

• SemesterItemResponseDto now exposes startDate/endDate so the frontend can eventually display academic dates on the switcher.

Out of scope (follow-ups)

• TieredPipelineSchedulerJob still reads QuestionnaireSubmission.createdAt for "new submissions" detection. Separate ticket for env kill-switch and/or submittedAt refactor.
• GET /enrollments/me has no semester filter — students will see enrollments from all semesters after S12526 backfill. Separate ticket for semesterId query param + "current academic semester" default.

PR

#383