feat(workers): sync.source_doc with sha256 diff

Fetches upstream URL via safe_fetch, sha256-diffs against the prior
body_sha stored in metadata, updates body_text + last_synced only when
content changed. Unchanged syncs just touch last_synced.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

workers/void_workers/handlers/__init__.py

@@ -1,8 +1,9 @@
-from . import echo, pdf, image, video
+from . import echo, pdf, image, video, sourcedoc
 
 REGISTRY = {
     echo.NAME: echo.handle,
     pdf.NAME: pdf.handle,
     image.NAME: image.handle,
     video.NAME: video.handle,
+    sourcedoc.NAME: sourcedoc.handle,
 }