Add per-course xSpacing/ySpacing for scaffold reset layout

[Copilot]

POST /api/course/scaffold/reset laid out top-level concepts using hardcoded pixel spacing (350 horizontal, 300 vertical), with no way for instructors to adjust it.

Backend

• Added xSpacing/ySpacing int columns to Course, defaulting to ConceptGraphService.MIN_HORIZONTAL_SEPARATION/VERTICAL_LEVEL_SEPARATION (350/300) — the same constants the layout algorithm already used
• Added Liquibase migration 042-add-spacing-to-course.json backfilling existing courses with those defaults
• Extended ConceptGraphService.reset(...) with an overload taking explicit xSpacing/ySpacing (existing overloads untouched, so default-spacing callers/tests are unaffected)
• POST /api/course/scaffold/reset now reads the course's spacing and passes it through instead of using the constants directly
• Added PUT /api/course/scaffold/spacing (courseId, xSpacing, ySpacing) to update these values, rejecting non-positive input
• Exposed xSpacing/ySpacing on InstructorCourseView so the frontend can show current values

Frontend

• ScaffoldTabComponent now fetches the course and renders editable X/Y Spacing fields with an "Update Spacing" button
• Shows a green ✓ on successful update, an inline error alert otherwise

conceptGraphService.reset(
    topLevelConcepts,
    edges,
    priorXByConceptId,
    course.getXSpacing(),
    course.getYSpacing());

[pconrad]

@copilot please resolve the merge conflicts.

[Copilot]

@copilot please resolve the merge conflicts.

Resolved in 97d79b9 — merged origin/main. The reset-scaffold button/mutation removed from ScaffoldTabComponent by the "move to footer" PR was dropped from this branch too (it's dead code here now); kept the spacing fields and updated their story/test mocks accordingly. Verified full frontend (npm test, 664 tests) and backend (mvn test, 81 classes) suites pass.

[pconrad]

@copilot please take care of the ci/ci failures

[Copilot]

@copilot please take care of the ci/ci failures

Fixed in 400d3... (pitest mutation coverage was 99%; a surviving mutant on the ySpacing <= 0 boundary check in updateScaffoldSpacing wasn't killed because the existing test used ySpacing=-1 instead of the boundary value 0). Added a test asserting ySpacing=0 is rejected — pitest now reports 100% mutation coverage. All 18 CI checks are green.