Make station price age informational only

The relay's price_updated is the maximum price_last_updated across the grades it maps, so it cannot vouch for the selected grade: stop using it to score, gate or warn about Best Value.

- FuelPriceFreshness is now station-level and display-only: one age per station, fresh/outdated/unknown/invalid, "May be outdated" past 30 days, with future-tolerance and non-finite guards.
- Best Value restores the closest-station-selling-the-selected-fuel baseline and its savings ranking; eligibility gating, eligible-first sorting and the unavailable banner are gone.
- Price age appears in the More info sheet only - no listing-row warnings, no extra banners.
- Copy reverted from "closest eligible station" to "closest station"; stale price-confidence strings removed, "May be outdated" added.
- Dormant per-grade timestamp plumbing removed, and regression tests added for every fuel type, missing/invalid/future stamps and the bundled offline dump.

Shared/FuelStore.swift

@@ -273,20 +273,13 @@ struct FuelStation: Identifiable, Codable, Equatable {
     let lng: Double
     /// Price per fuel type in pence per litre. Missing type = station doesn't sell it.
     let prices: [FuelType: Double]
-    /// Updated timestamp (seconds since 1970) — nil for sample data.
+    /// Station-level price timestamp (seconds since 1970): the relay's
+    /// `price_updated`, i.e. the newest price submission across the grades it
+    /// maps — never a per-grade time. nil when the source omits it (sample data).
     let priceUpdated: TimeInterval?
-    /// Per-grade timestamps when the source provides them. A station-wide
-    /// `price_updated` must not be copied into this map: it does not prove that
-    /// every grade changed at that time.
-    let priceUpdatedByFuel: [FuelType: TimeInterval]?
-    /// Original per-grade timestamp text, retained so malformed source values
-    /// cannot silently fall back to a station-wide timestamp.
-    let priceUpdatedByFuelRaw: [FuelType: String]?
-
     init(id: String, name: String, brand: String, address: String, postcode: String,
          lat: Double, lng: Double, prices: [FuelType: Double],
-         priceUpdated: TimeInterval?, priceUpdatedByFuel: [FuelType: TimeInterval]? = nil,
-         priceUpdatedByFuelRaw: [FuelType: String]? = nil) {
+         priceUpdated: TimeInterval?) {
         self.id = id
         self.name = name
         self.brand = brand
@@ -296,18 +289,12 @@ struct FuelStation: Identifiable, Codable, Equatable {
         self.lng = lng
         self.prices = prices
         self.priceUpdated = priceUpdated
-        self.priceUpdatedByFuel = priceUpdatedByFuel
-        self.priceUpdatedByFuelRaw = priceUpdatedByFuelRaw
     }
 
     private enum CodingKeys: String, CodingKey {
         case id, name, brand, address, postcode, lat, lng, prices
         case priceUpdated = "priceUpdated"
         case priceUpdatedSnake = "price_updated"
-        case priceUpdatedByFuel = "priceUpdatedByFuel"
-        case priceUpdatedByFuelSnake = "price_updated_by_fuel"
-        case priceUpdatedByFuelRaw = "priceUpdatedByFuelRaw"
-        case priceUpdatedByFuelRawSnake = "price_updated_by_fuel_raw"
     }
 
     init(from decoder: Decoder) throws {
@@ -322,10 +309,6 @@ struct FuelStation: Identifiable, Codable, Equatable {
         prices = try values.decode([FuelType: Double].self, forKey: .prices)
         priceUpdated = try values.decodeIfPresent(TimeInterval.self, forKey: .priceUpdated)
             ?? values.decodeIfPresent(TimeInterval.self, forKey: .priceUpdatedSnake)
-        priceUpdatedByFuel = try values.decodeIfPresent([FuelType: TimeInterval].self, forKey: .priceUpdatedByFuel)
-            ?? values.decodeIfPresent([FuelType: TimeInterval].self, forKey: .priceUpdatedByFuelSnake)
-        priceUpdatedByFuelRaw = try values.decodeIfPresent([FuelType: String].self, forKey: .priceUpdatedByFuelRaw)
-            ?? values.decodeIfPresent([FuelType: String].self, forKey: .priceUpdatedByFuelRawSnake)
     }
 
     func encode(to encoder: Encoder) throws {
@@ -339,8 +322,6 @@ struct FuelStation: Identifiable, Codable, Equatable {
         try values.encode(lng, forKey: .lng)
         try values.encode(prices, forKey: .prices)
         try values.encodeIfPresent(priceUpdated, forKey: .priceUpdated)
-        try values.encodeIfPresent(priceUpdatedByFuel, forKey: .priceUpdatedByFuel)
-        try values.encodeIfPresent(priceUpdatedByFuelRaw, forKey: .priceUpdatedByFuelRaw)
     }
 
     /// Cheap haversine distance to a location, in km.