Fix API base URL, rate-limit paging, background refresh; verify live against official API

- Base URL corrected to https://www.fuel-finder.service.gov.uk (the
  api.fuelfinder.service.gov.uk host in the portal's examples is stale —
  it does not resolve; www.fuel-finder... resolves to CloudFront and is
  what the live spec uses). Verified: /health now reports source=api,
  8,010 stations, 0 failures.
- Fuel Finder is strictly sequential (30 rpm per client, 429 on overlap):
  added FUEL_API_BATCH_SLEEP (default 4.0s) between pages ≈ 15 rpm.
- Stale-cache refresh moved off the request path into a background
  thread — a multi-minute rate-limited sync no longer blows the app's
  5s timeout; stale data is served while it catches up.
- Multiple diesel variants (B7_STANDARD, B7_PREMIUM, B10) now map to
  DIESEL as the MINIMUM price, so a premium price can't inflate the
  cheapest-diesel reference (CSV relay took first-wins; min is safer).
- Added B7_PREMIUM (underscore) to the fuel map — official key.
- scripts/run.sh now prefers the project venv (bare python3 missed
  uvicorn when run outside an activated env).
- Tests updated for min-diesel + B7_PREMIUM; 12/12 pass.

README.md

@@ -26,16 +26,21 @@ FuelBoard app / widget
 - Token refreshed automatically before expiry (60s buffer); reused across the
   two batched loops (docs: reuse tokens until near expiry).
 - Both endpoints paginated 500 records/page via `batch-number` until a short
-  page is returned.
+  page is returned; pages are spaced `FUEL_API_BATCH_SLEEP` (default 4s) apart
+  to stay inside the documented 30 rpm sequential-only rate limit.
 - Prices arrive as **decimal strings in pence** (`"0120.0000"` = 120.0p); some
   stations report pounds (values < 2.0 are multiplied by 100). Out-of-band
   values are dropped (50–500p sane band) so a pounds-denominated column can't
   poison the nationwide cheapest reference.
-- Fuel types mapped: `E5`/`E10` → same, `B7`/`B7_STANDARD`/`B7S`/`B7P`/`B10` →
-  `DIESEL`. Others (`HVO`, `SDV`, …) are ignored — the app tracks exactly
-  E5/E10/DIESEL.
-- Station snapshot cached `FUEL_CACHE_TTL` (default 300s); stale cache is
-  served on upstream failure so the app never sees a hard outage.
+- Fuel types mapped: `E5`/`E10` → same, `B7`/`B7_STANDARD`/`B7S`/`B7P`/
+  `B7_PREMIUM`/`B10` → `DIESEL`. Others (`HVO`, `SDV`, …) are ignored — the
+  app tracks exactly E5/E10/DIESEL.
+- Station snapshot cached `FUEL_CACHE_TTL` (default 300s); a stale cache is
+  refreshed in the background on the next request and stale data is served in
+  the meantime, so a multi-minute sync never blocks the app's 5s timeout.
+- Base URL is `https://www.fuel-finder.service.gov.uk` (NOT
+  `api.fuelfinder...` — that host doesn't resolve). Overridable per-endpoint
+  via env.
 - No credentials are exposed to clients — the app calls the relay keyless.
 
 ## Setup