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/config.py

@@ -1,5 +1,5 @@
 """Application configuration."""
 
-VERSION = "4.7.0"
+VERSION = "4.8.0"
 MAX_FILE_SIZE_MB = 10
 DEFAULT_COLOR = "#ffffff"