feat(backtest): saved strategy presets (strategy editor v1)

Persist named backtest-strategy presets so a staking config (bankroll,
min-score, stake rule, flat/percent/Kelly params) can be saved, listed,
loaded back into the form, and deleted. The per-run date range is not
part of a preset.

- Domain: SavedStrategy record (name trimmed + bounded to 80 chars,
  Create() factory) wrapping the pure BacktestStrategy.
- Persistence: SavedStrategyEntity + config (TEXT decimals, unique
  case-insensitive NOCASE index on Name), repository, mapping, and a
  hand-trimmed AddSavedStrategies migration (additive — only the new
  table). Case-insensitive names mean save-by-name overwrites instead of
  creating near-duplicates.
- Application: SaveStrategyUseCase (upsert by name, keeps Id+CreatedAt) +
  DeleteStrategyUseCase.
- UI: presets panel on the Backtest page (load/save/delete) + service
  methods; fraction<->percent round-trip; en/ru resx.
- Fix: pin Sports.Code as ValueGeneratedNever — it is the bookmaker's
  natural sport id, not an autoincrement surrogate. Corrects long-standing
  model-snapshot drift; the snapshot is regenerated to match the DB.
- 25 tests across all four layers: domain validation, real-SQLite
  round-trip incl. case-insensitive lookup/uniqueness, the upsert use
  case, and the service percent mapping.

src/Marathon.Infrastructure/Migrations/20260528225529_AddSavedStrategies.cs

@@ -0,0 +1,46 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Marathon.Infrastructure.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddSavedStrategies : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "SavedStrategies",
+                columns: table => new
+                {
+                    Id = table.Column<string>(type: "TEXT", nullable: false),
+                    Name = table.Column<string>(type: "TEXT", nullable: false, collation: "NOCASE"),
+                    StartingBankroll = table.Column<decimal>(type: "TEXT", nullable: false),
+                    MinScore = table.Column<decimal>(type: "TEXT", nullable: false),
+                    StakeRule = table.Column<int>(type: "INTEGER", nullable: false),
+                    FlatStake = table.Column<decimal>(type: "TEXT", nullable: false),
+                    PercentOfBankroll = table.Column<decimal>(type: "TEXT", nullable: false),
+                    KellyFraction = table.Column<decimal>(type: "TEXT", nullable: false),
+                    CreatedAt = table.Column<string>(type: "TEXT", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("PK_SavedStrategies", x => x.Id);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "IX_SavedStrategies_Name",
+                table: "SavedStrategies",
+                column: "Name",
+                unique: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "SavedStrategies");
+        }
+    }
+}