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.Application/UseCases/DeleteStrategyUseCase.cs

@@ -0,0 +1,26 @@
+using Marathon.Application.Abstractions;
+using Microsoft.Extensions.Logging;
+
+namespace Marathon.Application.UseCases;
+
+/// <summary>
+/// Removes a saved strategy preset by id. Silent no-op when the id is unknown.
+/// </summary>
+public sealed class DeleteStrategyUseCase
+{
+    private readonly ISavedStrategyRepository _repo;
+    private readonly ILogger<DeleteStrategyUseCase> _logger;
+
+    public DeleteStrategyUseCase(ISavedStrategyRepository repo, ILogger<DeleteStrategyUseCase> logger)
+    {
+        _repo = repo ?? throw new ArgumentNullException(nameof(repo));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+    }
+
+    public async Task ExecuteAsync(Guid id, CancellationToken ct = default)
+    {
+        await _repo.DeleteAsync(id, ct).ConfigureAwait(false);
+        await _repo.SaveChangesAsync(ct).ConfigureAwait(false);
+        _logger.LogInformation("DeleteStrategyUseCase: removed preset {Id}", id);
+    }
+}

src/Marathon.Application/UseCases/SaveStrategyUseCase.cs

@@ -0,0 +1,50 @@
+using Marathon.Application.Abstractions;
+using Marathon.Domain.Backtesting;
+using Microsoft.Extensions.Logging;
+
+namespace Marathon.Application.UseCases;
+
+/// <summary>
+/// Persists a named backtest-strategy preset. Upserts by name: saving under an
+/// existing name overwrites that preset's configuration (keeping its identity and
+/// original creation timestamp); a fresh name creates a new preset.
+/// </summary>
+public sealed class SaveStrategyUseCase
+{
+    private readonly ISavedStrategyRepository _repo;
+    private readonly ILogger<SaveStrategyUseCase> _logger;
+
+    public SaveStrategyUseCase(ISavedStrategyRepository repo, ILogger<SaveStrategyUseCase> logger)
+    {
+        _repo = repo ?? throw new ArgumentNullException(nameof(repo));
+        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+    }
+
+    /// <summary>Saves <paramref name="strategy"/> under <paramref name="name"/>.</summary>
+    /// <exception cref="ArgumentException">The name is empty or exceeds the length bound.</exception>
+    public async Task<SavedStrategy> ExecuteAsync(
+        string name, BacktestStrategy strategy, CancellationToken ct = default)
+    {
+        ArgumentNullException.ThrowIfNull(strategy);
+
+        // Validates + trims the name once, up front (throws ArgumentException if bad).
+        var candidate = SavedStrategy.Create(name, strategy);
+
+        var existing = await _repo.GetByNameAsync(candidate.Name, ct).ConfigureAwait(false);
+        if (existing is not null)
+        {
+            var updated = existing with { Strategy = strategy };
+            await _repo.UpdateAsync(updated, ct).ConfigureAwait(false);
+            await _repo.SaveChangesAsync(ct).ConfigureAwait(false);
+            _logger.LogInformation(
+                "SaveStrategyUseCase: overwrote preset {Name} ({Id})", updated.Name, updated.Id);
+            return updated;
+        }
+
+        await _repo.AddAsync(candidate, ct).ConfigureAwait(false);
+        await _repo.SaveChangesAsync(ct).ConfigureAwait(false);
+        _logger.LogInformation(
+            "SaveStrategyUseCase: created preset {Name} ({Id})", candidate.Name, candidate.Id);
+        return candidate;
+    }
+}