fix(pdf): flag scanned PDFs in pdf-to-text and serve text as UTF-8 (#603)

When a PDF has no text layer (scanned or image-only), pdf-to-text now
returns a 422 that points at the OCR tool instead of a silent empty file,
and text downloads carry charset=utf-8 so UTF-8 Arabic renders correctly
when the .txt is viewed inline.

Fixes #589

tests/helpers/python-gate.ts

@@ -22,7 +22,7 @@ function resolvePython(): string | null {
   return null;
 }
 
-const pythonBin = resolvePython();
+export const pythonBin = resolvePython();
 
 /** Check whether a Python module is importable by the resolved interpreter. */
 export function pythonWith(mod: string): boolean {