feat(replay): add BattleContextStore for start->finish handoff

Bridges the start-time -> finish-time gap. /finish carries neither
battle_id nor opponent identity; this store holds both for the finish
handler to compose a ViewerBattleHistory row.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.Database/Services/Replay/IBattleContextStore.cs

@@ -0,0 +1,17 @@
+namespace SVSim.Database.Services.Replay;
+
+/// <summary>
+/// In-memory per-viewer battle context store. Bridges the start-time → finish-time
+/// gap: the /finish request body carries neither battle_id nor opponent identity,
+/// so this stash holds everything the finish hook needs to compose a
+/// ViewerBattleHistory row.
+/// </summary>
+public interface IBattleContextStore
+{
+    /// <summary>Store the viewer's active battle context. Overwrites any prior entry.</summary>
+    void Set(long viewerId, BattleContext ctx);
+
+    /// <summary>Atomic read+clear. Returns null when no context (server restart,
+    /// non-tracked family, already taken). Finish handlers must tolerate null.</summary>
+    BattleContext? TakeFor(long viewerId);
+}