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

app/core/pdf_generator.py

@@ -609,7 +609,7 @@ def generate_pdf(doc) -> bytes:
             c.restoreState()
 
             # If block has a URL, make the entire block a clickable link
-            if getattr(block, 'url', None):
+            if block.url:
                 c.linkURL(block.url,
                           (bx + inset, row_y + inset,
                            bx + bw - inset, row_y + row_h - inset),