FAC-119 fix: correct semester year derivation and add category preview endpoint

[y4nder]

Problem

Two bugs in Moodle category provisioning plus a missing safety feature:

1. Wrong semester tag generation: ProvisionCategories extracted year values via .slice(2, 4) on startDate/endDate without accounting for the academic year. When a single semester is selected (dates fall in the same calendar year), this produces wrong tags — e.g., S22626 instead of S22526 for semester 2 of school year 2025-2026.

2. Unhelpful error on webservice_access_exception: The Moodle client threw a generic error with no guidance on how to fix the permission issue.

3. No category preview: Unlike courses, category provisioning went straight to execution with no preview step.

Solution

• Add ComputeSchoolYears() to MoodleCourseTransformService for school-year-aware startYY/endYY computation
• Replace static year extraction in ProvisionCategories with per-semester ComputeSchoolYears calls
• Improve webservice_access_exception error with actionable hint
• Add POST /moodle/provision/categories/preview endpoint (read-only, no concurrency guard)
• Add error wrapping to existing ProvisionCategories endpoint (consistency with tree endpoints)

Files Changed

• src/modules/moodle/services/moodle-course-transform.service.ts
• src/modules/moodle/services/moodle-course-transform.service.spec.ts
• src/modules/moodle/services/moodle-provisioning.service.ts
• src/modules/moodle/services/moodle-provisioning.service.spec.ts
• src/modules/moodle/lib/moodle.client.ts
• src/modules/moodle/lib/moodle.client.spec.ts (new)
• src/modules/moodle/controllers/moodle-provisioning.controller.ts
• src/modules/moodle/controllers/moodle-provisioning.controller.spec.ts

Testing

• 59 tests passing across 4 suites (27 new)
• Frontend admin console changes in separate admin.faculytics repo