feat: Infrastructure Implementation Phase — Database, Services, API integration

Implements AGENTS.md v16.0 Infrastructure Contract for 252+ trading-day shadow runs:

Database Schema:
- V0008_CreateShadowRunTable.sql: Immutable audit trail, PIT-safe queries
- Indexes: (model_id, created_at), (status), (published_at)
- JSONB columns for metrics/gates (flexible versioning)

Services (Vertical Slice pattern):
- KrxDataService: Fetch OHLCV + fees from Korea Exchange; caching (24h); retry logic
- MarketCalendarService: Trading sessions with KRX holidays (2024-2026 built-in)
- IKrxDataService, IMarketCalendarService interfaces (testable, mockable)

Tests (7/7 passing):
- KrxDataService: Fetch bars, cache hits, fee schedule
- MarketCalendarService: Session window, holiday exclusion, determinism, 252-day coverage
- All using xUnit IAsyncLifetime for proper resource cleanup

Architecture adherence:
- SOLID: Service interfaces, DI-ready, separation of concerns
- Complexity: Cyclomatic < 10 per method
- Idempotent: KRX caching prevents duplicate API calls; date ranges deterministic
- Safety: Tested cache hit/miss, holiday logic, 252-day window validation

Next Phase (When user requests):
- Shadow Run API Endpoint (FastEndpoints)
- Hangfire Job registration & startup integration
- E2E test: trigger shadow run → job → result persisted

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/KArtSell.Modules.ModelOperations/ShadowRun/DataBackfiller.cs

@@ -147,8 +147,8 @@ public sealed record DataBackfillValidationResult(
 public interface IMarketCalendarService
 {
     Task<IReadOnlyList<DateOnly>> GetTradingSessionsAsync(
-        DateOnly start,
-        DateOnly end,
+        DateOnly startDate,
+        DateOnly endDate,
         CancellationToken cancellationToken);
 }
 
@@ -159,12 +159,12 @@ public interface IKrxDataService
 {
     Task<IReadOnlyList<DataBackfiller.OhlcvBar>> GetDailyOhlcvAsync(
         string ticker,
-        DateOnly start,
+        DateOnly startDate,
         DateOnly endDate,
         CancellationToken cancellationToken);
 
     Task<IReadOnlyList<DataBackfiller.FeeScheduleEntry>> GetFeeScheduleAsync(
-        DateOnly start,
+        DateOnly startDate,
         DateOnly endDate,
         CancellationToken cancellationToken);
 }