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.

FuelBoard/SettingsView.swift

@@ -121,6 +121,33 @@ struct SettingsView: View {
                     Text("A small tip helps keep the data relay and app development going. Thank you!")
                 }
 
+                Section {
+                    HStack {
+                        Text("Connection")
+                        Spacer()
+                        Text(connectionText)
+                            .foregroundStyle(.secondary)
+                    }
+                    HStack {
+                        Text("Stations")
+                        Spacer()
+                        Text(stationCountText)
+                            .foregroundStyle(.secondary)
+                            .monospacedDigit()
+                    }
+                    HStack {
+                        Text("Data updated")
+                        Spacer()
+                        Text(dataUpdatedText)
+                            .foregroundStyle(.secondary)
+                            .monospacedDigit()
+                    }
+                } header: {
+                    Text("Data source")
+                } footer: {
+                    Text("Connection shows whether prices come from the official Fuel Finder API or the CSV mirror. Data updated is the latest price change reported by the GOV.UK server itself.")
+                }
+
                 Section {
                     HStack {
                         Text("Version")
@@ -252,6 +279,40 @@ struct SettingsView: View {
         return "\(version) (\(build))"
     }
 
+    /// "API" when the relay is serving the official Fuel Finder API, "CSV"
+    /// when it fell back to the public mirror, "—" when unknown/never fetched.
+    private var connectionText: String {
+        switch FuelStore.loadRelaySource()?.lowercased() {
+        case "api": return "API"
+        case "csv": return "CSV"
+        default: return "—"
+        }
+    }
+
+    private var stationCountText: String {
+        guard let count = FuelStore.loadStationCount() else { return "—" }
+        return count.formatted()
+    }
+
+    /// GOV.UK server's own dataset update timestamp (ISO 8601 from the API,
+    /// e.g. 2026-08-12T10:23:00.000Z). Shown as a local date/time — it is the
+    /// data's own freshness, not the relay's sync time.
+    private var dataUpdatedText: String {
+        guard let raw = FuelStore.loadDataUpdated(), !raw.isEmpty else { return "—" }
+        let iso = ISO8601DateFormatter()
+        iso.formatOptions = [.withInternetDateTime, .withFractionalSeconds]
+        if let date = iso.date(from: raw) {
+            return date.formatted(date: .abbreviated, time: .shortened)
+        }
+        // Some sources send "YYYY-MM-DDTHH:MM:SS" without fractional seconds
+        // or timezone — try the plain form before showing the raw string.
+        iso.formatOptions = [.withInternetDateTime]
+        if let date = iso.date(from: raw) {
+            return date.formatted(date: .abbreviated, time: .shortened)
+        }
+        return raw
+    }
+
     /// Hidden debug-mode toggle: five taps on the Version row flips the flag.
     /// Nothing in the UI advertises this — the Debug section simply appears or
     /// disappears. Deliberately NOT a visible switch so end users never see it.