epa_ppls: filter PPLS enumeration to row-crop products

The farmer-advisor consumer only cares about US row crops, so the EPA
scraper now drops products without at least one row-crop site in the
PPLS API response. Filter is on by default; --no-row-crop-filter
overrides for one-off broader pulls.

Filter shape:
  - Word-boundary regex match against each entry in the API's `sites`
    array (e.g., "SOYBEANS (FOLIAR TREATMENT)" → keep, "SHIPS, BOATS,
    SHIPHOLDS" → drop even though it contains "OATS" as substring).
  - Allowlist covers the major US row + small-grain + oilseed + sugar/
    fiber crops, plus alfalfa as a common rotation crop. See
    ROW_CROP_KEYWORDS in scrape/sources/epa_ppls.py for the full list.

Cost model:
  - 102K PPIS rows still need one API call each (no bulk filter
    available upstream), so enumeration still takes ~28h at 1 req/sec.
  - But PDF downloads drop from ~67K → ~5-10K (estimated row-crop
    hit rate), saving ~17h wall time and ~60GB disk on a full backfill.

Smoke test (4 mixed reg nos):
  524-475 Roundup Ultra        → kept (CORN/SOYBEANS/COTTON sites)
  524-591 Warrant              → kept (CORN/SOYBEANS/SORGHUM sites)
  100-1486 Advion Cockroach    → filtered (building/transport sites only)
  432-1276 (Bayer pet flea)    → filtered (no row crops)

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

sources.json

@@ -15,6 +15,7 @@
     "homepage": "https://ordspub.epa.gov/ords/pesticides/f?p=PPLS:1",
     "scraper": "scrape.sources.epa_ppls",
     "scraper_version": "0.1.0",
-    "license_note": "US federal government — public domain (no ToS restriction)"
+    "license_note": "US federal government — public domain (no ToS restriction)",
+    "scope_filter": "row-crop only — products with at least one site matching CORN, MAIZE, POPCORN, SOYBEAN(S), COTTON, WHEAT, RICE, SORGHUM, MILO, BARLEY, OATS, RYE, SUNFLOWER(S), PEANUT(S), SUGAR BEET(S), DRY/FIELD BEAN(S), CANOLA, RAPESEED, or ALFALFA (word-boundary match). Pass --no-row-crop-filter to scrape the full PPLS universe."
   }
 ]