fix: URL persistence in export + cache-busting v4.8.0

- getDocument() now explicitly lists all block fields including url
- loadDocument() initializes url/series_id for backward compat with older JSONs
- Cache-busting query params (?v=4.8) on all static assets
- PDF generator uses block.url directly (proper model field)
- Added 7 new URL tests (model, API, PDF link annotations)
- Version bumped to 4.8.0

tests/test_api.py

@@ -185,3 +185,23 @@ def test_backward_compat_date_field(client):
     r = client.post("/api/validate", json=doc)
     assert r.status_code == 200
     assert r.json()["valid"] is True
+
+
+def test_generate_pdf_with_url_block(client):
+    """PDF generation should succeed when blocks contain url field."""
+    doc = make_valid_doc()
+    doc["blocks"][0]["url"] = "https://example.com/linked"
+    r = client.post("/api/generate-pdf", json=doc)
+    assert r.status_code == 200
+    assert r.content[:5] == b'%PDF-'
+    # Verify link annotation is present in the PDF
+    assert b'example.com/linked' in r.content
+
+
+def test_validate_accepts_url_field(client):
+    """Blocks with or without url should both validate successfully."""
+    doc = make_valid_doc()
+    doc["blocks"][0]["url"] = "https://example.com"
+    r = client.post("/api/validate", json=doc)
+    assert r.status_code == 200
+    assert r.json()["valid"] is True