feat(sources): PDF-Upload als neuer Quellentyp pdf_document

- POST /api/sources/global/upload-pdf: multipart File-Upload,
  50 MB Limit, SHA256-Dedup, speichert PDF unter <dirname(DB)>/pdfs/{sha}.pdf,
  legt Source mit processed_at=NULL an (Monitor verarbeitet asynchron)
- pattern in GlobalSourceUpdate um pdf_document erweitert (2x)
- dashboard.html: Button + Modal im Grundquellen-Sub-Tab
- sources.js: openPdfUploadModal + setupPdfUploadForm + FormData-Submit
- app.js: API.upload(path, formData) Helper fuer multipart
- requirements.txt: pypdf (Validierung optional)

src/static/js/app.js

@@ -26,6 +26,23 @@ const API = {
   post(path, body) { return this.request(path, { method: "POST", body: JSON.stringify(body) }); },
   put(path, body) { return this.request(path, { method: "PUT", body: body ? JSON.stringify(body) : undefined }); },
   del(path) { return this.request(path, { method: "DELETE" }); },
+
+  async upload(path, formData) {
+    const headers = {};
+    if (this.token) headers["Authorization"] = `Bearer ${this.token}`;
+    const res = await fetch(path, { method: "POST", headers, body: formData });
+    if (res.status === 401) {
+      localStorage.removeItem("token");
+      localStorage.removeItem("username");
+      window.location.href = "/";
+      return;
+    }
+    if (!res.ok) {
+      const data = await res.json().catch(() => ({}));
+      throw new Error(data.detail || `Fehler ${res.status}`);
+    }
+    return res.json();
+  },
 };
 
 // --- State ---