feat: add Sign PDF tool (draw/type/upload signatures, place on a PDF) (#370)

Draw, type, or upload a signature and place resizable/rotatable copies across PDF pages; output flattened server-side with PyMuPDF. Visual electronic signature, not cryptographic. New interactive-sign display mode (pdf.js + Konva) and a custom docs-pool route.

packages/shared/src/constants.ts

@@ -1580,6 +1580,18 @@ const BASE_TOOLS: Tool[] = [
     acceptedInputs: [".pdf"],
     executionHint: "fast",
   },
+  {
+    id: "sign-pdf",
+    name: "Sign PDF",
+    description: "Draw, type, or upload a signature and place it anywhere on a PDF",
+    category: "pdf-edit",
+    icon: "Signature",
+    route: "/sign-pdf",
+    modality: "document",
+    acceptedInputs: [".pdf"],
+    executionHint: "fast",
+    keywords: ["sign", "signature", "e-sign", "esign", "fill and sign", "autograph"],
+  },
   {
     id: "pdf-to-text",
     name: "PDF to Text",