Harden owner and suggestion invariants for concurrent writes

Data/Migrations/20260208203323_HardenOwnerAndSuggestionInvariants.cs

@@ -0,0 +1,47 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace GameList.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class HardenOwnerAndSuggestionInvariants : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateIndex(
+                name: "IX_Players_IsOwner",
+                table: "Players",
+                column: "IsOwner",
+                unique: true,
+                filter: "IsOwner = 1");
+
+            migrationBuilder.Sql(
+                """
+                CREATE TRIGGER IF NOT EXISTS TR_Suggestions_MaxFivePerPlayer
+                BEFORE INSERT ON Suggestions
+                WHEN
+                    (SELECT COUNT(1) FROM Suggestions WHERE PlayerId = NEW.PlayerId) >= 5
+                    AND (
+                        COALESCE((SELECT HasJoker FROM Players WHERE Id = NEW.PlayerId), 0) = 0
+                        OR COALESCE((SELECT CurrentPhase FROM Players WHERE Id = NEW.PlayerId), 0) != 2
+                    )
+                BEGIN
+                    SELECT RAISE(ABORT, 'suggestion_limit_exceeded');
+                END;
+                """
+            );
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.Sql("DROP TRIGGER IF EXISTS TR_Suggestions_MaxFivePerPlayer;");
+
+            migrationBuilder.DropIndex(
+                name: "IX_Players_IsOwner",
+                table: "Players");
+        }
+    }
+}