perf: replace in-memory pagination with DB-level keyset queries

[Tobi-8]

• Add findPaginated(limit, afterId) to user and course repositories using WHERE id > :afterId LIMIT n+1 keyset pattern
• Update findBySearchTerm, findByCategory, findByDifficulty, findByCategoryAndDifficulty to accept limit/afterId params
• Rewrite ListUsersUseCase and ListCoursesUseCase to delegate pagination to the repository instead of slicing in memory
• Add USER_REPOSITORY and COURSE_REPOSITORY string tokens for NestJS DI (interfaces cannot be used as runtime tokens)
• Wire use cases, repositories, mappers, and persistence entities into UserModule and CourseModule providers
• Add safety take(1000) cap on raw findAll() calls
• Remove unreachable throw in auth.service.ts resetPassword

closes #783

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