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.DbMigrator/0008_CreateShadowRunTable.sql

@@ -0,0 +1,65 @@
+-- Migration: Create shadow_run table for 252+ trading-day model validation
+-- Purpose: Immutable append-only audit trail for shadow run results
+-- PIT Safety: published_at column enables point-in-time queries
+-- Idempotency: Schema exists → no-op; checksum validation prevents duplicate runs
+
+CREATE SCHEMA IF NOT EXISTS model_operations;
+
+CREATE TABLE IF NOT EXISTS model_operations.shadow_run (
+    run_id UUID PRIMARY KEY,
+    model_id UUID NOT NULL,
+    window_start DATE NOT NULL,
+    window_end DATE NOT NULL,
+    status VARCHAR(50) NOT NULL DEFAULT 'Pending',
+
+    -- Performance metrics (JSONB for flexible schema versioning)
+    metrics_json JSONB,
+
+    -- Phase breakdown: Bull, Bear, Sideways, Volatility
+    phase_analysis_json JSONB,
+
+    -- Cost scenario analysis
+    cost_analysis_json JSONB,
+
+    -- False exit / reentry attribution
+    false_exit_analysis_json JSONB,
+
+    -- Validation gates: PBO ≤ 20%, DSR ≥ 95%, cost 2x positive
+    validation_gates_json JSONB,
+
+    -- Error context if status = Failed
+    error_message TEXT,
+
+    -- Audit timestamps
+    created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    published_at TIMESTAMP,  -- NULL = unpublished; populated when final
+
+    CONSTRAINT check_window_order CHECK (window_start <= window_end),
+    CONSTRAINT check_status CHECK (status IN ('Pending', 'DataBackfill', 'Replay', 'EvaluationComplete', 'Failed'))
+);
+
+-- Indexes for common queries
+CREATE INDEX IF NOT EXISTS idx_shadow_run_model_created
+    ON model_operations.shadow_run (model_id, created_at DESC);
+
+CREATE INDEX IF NOT EXISTS idx_shadow_run_status
+    ON model_operations.shadow_run (status);
+
+CREATE INDEX IF NOT EXISTS idx_shadow_run_published_at
+    ON model_operations.shadow_run (published_at);
+
+-- Table comments for documentation
+COMMENT ON TABLE model_operations.shadow_run IS
+    '252+ trading-day model validation runs. Append-only immutable audit trail. PIT-safe: queries use published_at <= cutoff.';
+
+COMMENT ON COLUMN model_operations.shadow_run.run_id IS
+    'Unique shadow run identifier. Idempotency key for job deduplication.';
+
+COMMENT ON COLUMN model_operations.shadow_run.status IS
+    'Execution phase: Pending → DataBackfill → Replay → EvaluationComplete or Failed.';
+
+COMMENT ON COLUMN model_operations.shadow_run.published_at IS
+    'Timestamp when results finalized. NULL = unpublished. Used for PIT queries (published_at <= @cutoff).';
+
+COMMENT ON COLUMN model_operations.shadow_run.validation_gates_json IS
+    'Production readiness gates: {pbo_under_20: bool, dsr_above_95: bool, cost_2x_positive: bool, all_gates_passed: bool}';