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/README.md

@@ -41,6 +41,23 @@ Every scraper is **idempotent** by default — re-running with the
 same arguments skips records already on disk. Use `--force` to
 re-fetch.
 
+### Corpus location
+
+Default: `corpus/` at the repo root. Override with the
+`PPLS_CORPUS_ROOT` env var to route the corpus to external storage
+(USB drive, NAS mount, secondary partition):
+
+```bash
+export PPLS_CORPUS_ROOT=/mnt/big-disk/ppls-corpus
+python -m scrape.runner --source bayer --limit 20
+# writes to /mnt/big-disk/ppls-corpus/bayer/...
+```
+
+All sources honor the same env var; each creates its own
+`<source_id>/` subdirectory beneath it. Per-source code paths
+still resolve `CORPUS_DIR` correctly whether the env var is set
+or not.
+
 ## Scope: corn / soybeans / wheat
 
 The corpus is scoped to the three crops the consumer app focuses on: