Envelope metadata: source, stations_count, data_updated in stations + health

- /api/v1/stations now returns source (api|csv), stations_count, and
  data_updated = freshest price_last_updated across the dataset, i.e. the
  GOV.UK server's own update time (not the relay's sync time).
- /health gains data_updated too.
- Tracked during _fetch_api (max price_last_updated) and _fetch_csv (max
  price_change_effective_timestamp).

app/main.py

@@ -95,6 +95,7 @@ _state = {
     "sync_attempts": 0,
     "sync_failures": 0,
     "source_used": None,
+    "data_updated": None,      # freshest govUK price_last_updated across dataset
 }
 
 # ---------------------------------------------------------------------------
@@ -299,12 +300,24 @@ def _parse_csv_to_stations(text: str) -> list:
 
 def _fetch_csv() -> list:
     """Download the hourly full-UK CSV mirror."""
+    import csv
+    import io
+
     with httpx.Client(timeout=120) as client:
         resp = client.get(CSV_URL)
         resp.raise_for_status()
         stations = _parse_csv_to_stations(resp.text)
         if not stations:
             raise RuntimeError("CSV parse returned zero stations")
+        # Freshest price-change timestamp in the CSV dump (govUK-side update
+        # time for the mirror's source data).
+        latest = None
+        for row in csv.DictReader(io.StringIO(resp.text)):
+            for key, val in row.items():
+                if key.startswith("forecourts.price_change_effective_timestamp."):
+                    if val and (latest is None or val > latest):
+                        latest = val
+        _state["data_updated"] = latest
         return stations
 
 
@@ -383,6 +396,15 @@ def _fetch_api() -> list:
         stations = _normalise_api_payload(stations_info, fuel_prices)
         if not stations:
             raise RuntimeError("API fetch returned zero stations")
+        # Freshest price timestamp the GOV.UK server itself reports — this is
+        # the dataset's own update time, independent of when we synced.
+        latest = None
+        for rec in fuel_prices:
+            for entry in rec.get("fuel_prices") or []:
+                ts = entry.get("price_last_updated")
+                if ts and (latest is None or ts > latest):
+                    latest = ts
+        _state["data_updated"] = latest
         return stations
 
 
@@ -504,6 +526,7 @@ def health():
         "source": _state["source_used"],
         "stations_cached": _state["stations"] is not None,
         "stations_count": len(_stations_list()),
+        "data_updated": _state["data_updated"],
         "last_sync": time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime(_state["stations_fetched_at"])) if _state["stations_fetched_at"] else None,
         "sync_attempts": _state["sync_attempts"],
         "sync_failures": _state["sync_failures"],
@@ -565,7 +588,14 @@ def stations(
     else:
         rows.sort(key=lambda r: r["price"] if r["price"] is not None else float("inf"))
 
-    return {"fuel": fuel, "count": len(rows[:limit]), "stations": rows[:limit]}
+    return {
+        "fuel": fuel,
+        "count": len(rows[:limit]),
+        "stations_count": len(rows),
+        "source": _state["source_used"],
+        "data_updated": _state["data_updated"],
+        "stations": rows[:limit],
+    }
 
 
 if __name__ == "__main__":