feat(courses): add GET /api/v1/courses/:id/syllabus endpoint (#373)

[ZacLou]

Summary

Add a GET /api/v1/courses/:id/syllabus endpoint that returns the full course syllabus — module order, title, description, estimated duration, and learning objectives — as requested in #373.

What changed

• course.types.ts: Added SyllabusModule and CourseSyllabus interfaces.
• course.service.ts: Added getSyllabus(courseId) — builds an ordered module list from courses.courseModules jsonb (falls back to quiz-derived module IDs when rich metadata isn't present), derives learning objectives from each module's quiz question prompts (first 5 per module to keep the list focused), computes total estimated duration, and caches the result for 5 minutes per the issue's acceptance criteria.
• course.controller.ts: Added syllabus() controller method (thin passthrough).
• course.routes.ts: Registered GET /:id/syllabus with courseIdParamsSchema validation and Swagger documentation.

Acceptance criteria checklist

• Returns complete syllabus with descriptions
• Includes estimated duration per module
• Shows learning objectives (derived from quiz question prompts)
• Cached for 5 minutes

Technical notes

• Module data comes from courseModules jsonb (priority) or falls back to quiz moduleId groups — same fallback strategy getCourseDetail already uses.
• Learning objectives are derived from quiz question prompts where available; modules without quizzes return an empty learningObjectives array.
• totalEstimatedDurationMinutes is null when no module has a duration set, otherwise the sum of all module durations.

Closes #373