feat(workers): extract.pdf with Tesseract fallback

pdftotext first; falls back to per-page pdftoppm rasterization +
Tesseract OCR when the extracted text is < 200 chars. Updates
refs.body_text + metadata.extract.{method,chars} via the repo shim;
audit entry emitted with actor_kind='worker'.

born_digital.pdf fixture padded so pdftotext yields > 200 chars and
the test exercises the pdftotext path, not the OCR fallback.

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

workers/void_workers/handlers/__init__.py

@@ -1,5 +1,6 @@
-from . import echo
+from . import echo, pdf
 
 REGISTRY = {
     echo.NAME: echo.handle,
+    pdf.NAME: pdf.handle,
 }