Make PriceCharting runs auditable and pin matches by product id

Two changes aimed at the first real run, since the integration cannot be
exercised here without a subscription.

Refresh now reports which product each game matched — name, console and the
source's id — next to the prices, and dry-run surfaces it before anything is
written. This is the failure that would otherwise go unnoticed: a lookup for
the DS "Chrono Trigger" resolving to the SNES original returns entirely
plausible numbers for the wrong game, and nothing in a bare price would say
so.

The matched id is then stored on the game, and later refreshes look it up
directly instead of repeating the title search. Cheaper, and stable — a
search that drifts to a different edition next month cannot silently
re-price something that was already matched correctly.

IPriceProvider takes an optional sourceId so this stays provider-agnostic.
eBay ignores it, having no stable per-product identifier in Browse.

139 backend tests.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/src/LudosData.Api/Domain/Game.cs

@@ -94,6 +94,15 @@ public class Game
     // re-prices it without another lookup, and the dashboard can answer both
     // "what is this worth" and "what would it be worth complete".
 
+    /// <summary>
+    /// The price source's identifier for this game, kept after the first match.
+    /// Later refreshes look it up directly instead of repeating a fuzzy search,
+    /// which makes them both cheaper and stable — a title search that drifts to
+    /// a different edition next month would silently re-price the wrong thing.
+    /// </summary>
+    [MaxLength(100)]
+    public string? PriceSourceId { get; set; }
+
     public decimal? ValueLoose { get; set; }
     public decimal? ValueCib { get; set; }
     public decimal? ValueNew { get; set; }