Add server-side search, filtering, sorting and pagination

[chrisdedman]

Description

Current filtering is suitable while the directory is small, but downloading every project and filtering in Vue won't scale indefinitely.

Proposed API

GET /api/projects?q=moderation
                  &category=Utility
                  &sort=recent
                  &page=1
                  &page_size=24

Acceptance criteria

• query search
• category filtering
• pagination
• deterministic sorting
• total result count
• frontend URL reflects filters