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_core.py

@@ -22,6 +22,7 @@ def test_block_optional_fields():
     b = Block(date="2026-03-01", start="09:00", end="10:00", title="Test", type_id="ws")
     assert b.responsible is None
     assert b.notes is None
+    assert b.url is None
     assert b.series_id is None
 
 
@@ -31,6 +32,26 @@ def test_block_series_id():
     assert b.series_id == "s_abc123"
 
 
+def test_block_with_url():
+    b = Block(date="2026-03-01", start="09:00", end="10:00", title="Test", type_id="ws",
+              url="https://example.com/test")
+    assert b.url == "https://example.com/test"
+
+
+def test_block_url_in_serialization():
+    """url field must appear in serialized JSON even when None."""
+    b = Block(id="b1", date="2026-03-01", start="09:00", end="10:00",
+              title="Test", type_id="ws")
+    data = b.model_dump(mode="json")
+    assert "url" in data
+    assert data["url"] is None
+
+    b2 = Block(id="b2", date="2026-03-01", start="09:00", end="10:00",
+               title="Test", type_id="ws", url="https://example.com")
+    data2 = b2.model_dump(mode="json")
+    assert data2["url"] == "https://example.com"
+
+
 def test_block_with_all_fields():
     b = Block(
         id="custom-id", date="2026-03-01", start="09:00", end="10:00",