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>

src/web/server.ts

@@ -515,11 +515,10 @@ export class WebServer extends EventEmitter {
   }
 
   private async setupRoutes(): Promise<void> {
-    // Allow multipart/form-data for screenshot uploads — skip Fastify's body parser
-    // so the route handler can read the raw stream directly.
-    this.app.addContentTypeParser('multipart/form-data', (_req, _payload, done) => {
-      done(null);
-    });
+    // multipart/form-data: parser is provided by @fastify/multipart (registered
+    // below). Its parser is a no-op marker that leaves the body on req.raw, so
+    // legacy routes that read the raw stream directly (e.g. /api/screenshots)
+    // continue to work alongside routes that use req.file() (e.g. paste-image).
 
     // Enable gzip/brotli compression for all responses.
     // Massive win: 793KB uncompressed → ~120KB compressed for static assets.