Projects: create project tied to the user (owner membership)

Scope: POST /api/sites sets ownerId = current user + creates an owner membership. Generate the analytics site + the error-tracking API key on creation.

Done when: a created project is owned by its creator and immediately usable (script tag + DSN available).

Implemented: handleCreateSite ties the project to the creator — sets ownerId + OWNER# GSI, records an owner membership (addMembership, #55), and auto-provisions the default API key. Project CRUD is now complete: create (#57), list owned+shared (#56), delete (#58).