FuelBoard Relay API: official Fuel Finder API adapter (sibling of CSV relay)

- Talks to the OFFICIAL GOV.UK Fuel Finder API instead of the public CSV
  mirror: OAuth token (/api/v1/oauth/generate_access_token), batched
  paging (500/page via batch-number) over /api/v1/pfs and
  /api/v1/pfs/fuel-prices, joined on node_id.
- Price strings parsed to pence with pounds->pence correction (< 2.0);
  50-500p sane band keeps junk out of the cheapest reference.
- Fuel mapping: E5/E10 as-is; B7/B7_STANDARD/B7S/B7P/B10 -> DIESEL;
  HVO/SDV dropped (app tracks E5/E10/DIESEL only).
- lat/lng parsed from nested location strings.
- Output contract IDENTICAL to fuelboard-relay (/health + /api/v1/stations),
  so the app/widget/alerts work unchanged; switch = point app at :8789.
- CSV mirror kept as automatic fallback (FUEL_SOURCE=auto) so the full
  path is testable before GOV.UK One Login creds exist.
- 12 unit tests against documented API shapes (no network).

.env.example

@@ -0,0 +1,9 @@
+# Copy to .env and fill in — NEVER commit .env
+FUEL_API_CLIENT_ID=
+FUEL_API_CLIENT_SECRET=
+FUEL_API_TOKEN_URL=https://api.fuelfinder.service.gov.uk/api/v1/oauth/generate_access_token
+FUEL_API_PFS_URL=https://api.fuelfinder.service.gov.uk/api/v1/pfs
+FUEL_API_PRICES_URL=https://api.fuelfinder.service.gov.uk/api/v1/pfs/fuel-prices
+FUEL_CACHE_TTL=300
+FUEL_RELAY_PORT=8789
+# FUEL_SOURCE=auto   # auto (API if creds, CSV fallback) | csv | api