fix(server): remove duplicate multipart parser conflicting with @fastify/multipart

#90 added @fastify/multipart, which registers its own multipart/form-data
content-type parser. Combined with the existing manual no-op parser in
setupRoutes() (originally there so /api/screenshots could read req.raw
directly), this raises "Content type parser 'multipart/form-data' already
present" at server boot and the process exits. CI did not catch it
because ci.yml runs typecheck + lint only.

@fastify/multipart's parser is a no-op marker (sets req[kMultipart] =
true and returns) and leaves the body on req.raw, so the legacy
/api/screenshots handler that reads req.raw directly keeps working
unchanged. The manual parser was redundant the moment the plugin was
registered.

Smoke-tested locally: server boots, /api/sessions/:id/paste-image
returns 200 / 403-CSRF / 415-magic-mismatch / 413-oversize / 429-rate
as designed; /api/screenshots upload still returns 200.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>