scrape: route corpus via PPLS_CORPUS_ROOT env var

Both scrapers now honor PPLS_CORPUS_ROOT so the corpus can land on
external storage (USB drive, NAS mount, secondary partition) without
editing the repo. Default behavior unchanged: corpus/ at repo root
when the env var is unset.

Per-source subdirectory layout preserved: ${PPLS_CORPUS_ROOT}/bayer/,
${PPLS_CORPUS_ROOT}/epa_ppls/, etc.

Live-verified against /run/media/justin/USB (vfat, 59GB free):
  PPLS_CORPUS_ROOT=/run/media/justin/USB/ppls-corpus \
    python -m scrape.runner --source epa_ppls --reg-no 524-475
  -> wrote to USB, root disk untouched

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scrape/sources/bayer.py

@@ -63,8 +63,12 @@ PRODUCT_CLASS = {
 }
 
 # Repo root: scrape/sources/bayer.py -> repo root is 3 parents up.
+# Corpus root is overridable via PPLS_CORPUS_ROOT for routing the
+# corpus to external storage (USB drive, NAS mount, etc.) without
+# editing the repo.
 REPO_ROOT = Path(__file__).resolve().parents[2]
-CORPUS_DIR = REPO_ROOT / "corpus" / "bayer"
+CORPUS_ROOT = Path(os.environ.get("PPLS_CORPUS_ROOT") or REPO_ROOT / "corpus")
+CORPUS_DIR = CORPUS_ROOT / "bayer"
 
 # Politeness: target ~1 req/sec to Bayer. Each HTTP method goes through
 # a tiny token-bucket sleeper to enforce this without per-call asyncio.