Settings About: data source section (connection, station count, GOV.UK update time)

- Relay envelope now carries source (api|csv), stations_count, and
  data_updated (freshest price_last_updated the GOV.UK server reports,
  independent of relay sync time).
- App decodes the new envelope metadata (RelayMeta), persists it after
  each full fetch, and shows it in Settings → About → Data source:
  Connection (API/CSV), Stations count, Data updated (local-formatted).
- Older relays without the fields decode cleanly (nil meta → em-dash).
- 2 new tests for meta decode + absent-meta tolerance; 37/37 pass.

FuelBoardTests/Tests/FuelBoardSharedTests/FuelBoardTests.swift

@@ -62,6 +62,27 @@ final class PriceGuardTests: XCTestCase {
         XCTAssertEqual(stations[0].prices[.diesel], 142.9)
     }
 
+    func testRelayMetaDecoded() throws {
+        let json = """
+        {"source":"api","stations_count":8010,"data_updated":"2026-08-12T10:23:00.000Z","stations":[{"id":"s1","name":"SHELL LEEDS","brand":"Shell","address":"A1","postcode":"LS1 1AA","lat":53.8,"lng":-1.5,"prices":{"E10":137.9}}]}
+        """
+        let meta = FuelPriceProvider.decodeRelayMeta(from: Data(json.utf8))
+        XCTAssertEqual(meta?.source, "api")
+        XCTAssertEqual(meta?.stationCount, 8010)
+        XCTAssertEqual(meta?.dataUpdated, "2026-08-12T10:23:00.000Z")
+    }
+
+    func testRelayMetaAbsentIsNil() throws {
+        // Older relay without envelope metadata must not fail decode — nil meta.
+        let json = """
+        {"stations":[{"id":"s1","name":"SHELL LEEDS","brand":"Shell","address":"A1","postcode":"LS1 1AA","lat":53.8,"lng":-1.5,"prices":{"E10":137.9}}]}
+        """
+        let meta = FuelPriceProvider.decodeRelayMeta(from: Data(json.utf8))
+        XCTAssertNil(meta?.source)
+        XCTAssertNil(meta?.stationCount)
+        XCTAssertNil(meta?.dataUpdated)
+    }
+
     func testOutOfBandPricesDropped() throws {
         let json = """
         {"stations":[{"id":"s1","name":"GARBAGE","brand":"X","address":"A","postcode":"L","lat":0,"lng":0,"prices":{"E10":1.3,"E5":1589.0,"DIESEL":137.9}}]}