fix: downgrade pdf-parse to 1.1.1 to eliminate native dependency

pdf-parse@2.4.5 depends on @napi-rs/canvas (native Skia binary compiled
with AVX instructions) which crashes with SIGILL on the Synology NAS CPU.
Version 1.1.1 is pure JavaScript and works on any architecture.

backend/package.json

@@ -18,7 +18,7 @@
     "express": "^5.2.1",
     "multer": "^2.1.1",
     "openai": "^6.27.0",
-    "pdf-parse": "^2.4.5",
+    "pdf-parse": "1.1.1",
     "pg": "^8.19.0",
     "uuid": "^13.0.0"
   },