fix(r3): a NUL in the raw file path is a 404, not a 500

Compare's stages load their pictures through the catch-all file route, which
caught only OSError around resolve(); an embedded NUL raises ValueError. Same
class as resolve_booth's fix in f8d136a (heid bug hunt on the race fix,
hulda). The upload route's NUL-in-filename 500 is the same class and is left
to booth-dev: it is not on compare's path.

booth/app.py

@@ -2148,7 +2148,9 @@ def create_app(
         booth = resolve_booth(name)
         try:
             target = (booth / filepath).resolve()
-        except OSError:
+        except (OSError, ValueError):
+            # ValueError: an embedded NUL (`/b/g/p%00.png`) is not an OSError,
+            # and hostile input is a 404, never a 500 (r3 heid bug hunt)
             raise HTTPException(status_code=404, detail="no such file")
         if not str(target).startswith(str(booth) + os.sep) or not target.is_file():
             raise HTTPException(status_code=404, detail="no such file")