feat: transform to REST API service with SQLite persistence (v0.3.0)

Major architecture transformation from batch-only to API service with
database persistence for Windmill integration.

## REST API Implementation
- POST /simulate/trigger - Start simulation jobs
- GET /simulate/status/{job_id} - Monitor job progress
- GET /results - Query results with filters (job_id, date, model)
- GET /health - Service health checks

## Database Layer
- SQLite persistence with 6 tables (jobs, job_details, positions,
  holdings, reasoning_logs, tool_usage)
- Foreign key constraints with cascade deletes
- Replaces JSONL file storage

## Backend Components
- JobManager: Job lifecycle management with concurrency control
- RuntimeConfigManager: Thread-safe isolated runtime configs
- ModelDayExecutor: Single model-day execution engine
- SimulationWorker: Date-sequential, model-parallel orchestration

## Testing
- 102 unit and integration tests (85% coverage)
- Database: 98% coverage
- Job manager: 98% coverage
- API endpoints: 81% coverage
- Pydantic models: 100% coverage
- TDD approach throughout

## Docker Deployment
- Dual-mode: API server (persistent) + batch (one-time)
- Health checks with 30s interval
- Volume persistence for database and logs
- Separate entrypoints for each mode

## Validation Tools
- scripts/validate_docker_build.sh - Build validation
- scripts/test_api_endpoints.sh - Complete API testing
- scripts/test_batch_mode.sh - Batch mode validation
- DOCKER_API.md - Deployment guide
- TESTING_GUIDE.md - Testing procedures

## Configuration
- API_PORT environment variable (default: 8080)
- Backwards compatible with existing configs
- FastAPI, uvicorn, pydantic>=2.0 dependencies

Co-Authored-By: AI Assistant <noreply@example.com>

tests/conftest.py

@@ -0,0 +1,140 @@
+"""
+Shared pytest fixtures for AI-Trader API tests.
+
+This module provides reusable fixtures for:
+- Test database setup/teardown
+- Mock configurations
+- Test data factories
+"""
+
+import pytest
+import tempfile
+import os
+from pathlib import Path
+from api.database import initialize_database, get_db_connection
+
+
+@pytest.fixture(scope="session")
+def test_db_path():
+    """Create temporary database file for testing session."""
+    temp_db = tempfile.NamedTemporaryFile(delete=False, suffix=".db")
+    temp_db.close()
+
+    yield temp_db.name
+
+    # Cleanup after all tests
+    try:
+        os.unlink(temp_db.name)
+    except FileNotFoundError:
+        pass
+
+
+@pytest.fixture(scope="function")
+def clean_db(test_db_path):
+    """
+    Provide clean database for each test function.
+
+    This fixture:
+    1. Initializes schema if needed
+    2. Clears all data before test
+    3. Returns database path
+
+    Usage:
+        def test_something(clean_db):
+            conn = get_db_connection(clean_db)
+            # ... test code
+    """
+    # Ensure schema exists
+    initialize_database(test_db_path)
+
+    # Clear all tables
+    conn = get_db_connection(test_db_path)
+    cursor = conn.cursor()
+
+    # Delete in correct order (respecting foreign keys)
+    cursor.execute("DELETE FROM tool_usage")
+    cursor.execute("DELETE FROM reasoning_logs")
+    cursor.execute("DELETE FROM holdings")
+    cursor.execute("DELETE FROM positions")
+    cursor.execute("DELETE FROM job_details")
+    cursor.execute("DELETE FROM jobs")
+
+    conn.commit()
+    conn.close()
+
+    return test_db_path
+
+
+@pytest.fixture
+def sample_job_data():
+    """Sample job data for testing."""
+    return {
+        "job_id": "test-job-123",
+        "config_path": "configs/test.json",
+        "status": "pending",
+        "date_range": '["2025-01-16", "2025-01-17"]',
+        "models": '["gpt-5", "claude-3.7-sonnet"]',
+        "created_at": "2025-01-20T14:30:00Z"
+    }
+
+
+@pytest.fixture
+def sample_position_data():
+    """Sample position data for testing."""
+    return {
+        "job_id": "test-job-123",
+        "date": "2025-01-16",
+        "model": "gpt-5",
+        "action_id": 1,
+        "action_type": "buy",
+        "symbol": "AAPL",
+        "amount": 10,
+        "price": 255.88,
+        "cash": 7441.2,
+        "portfolio_value": 10000.0,
+        "daily_profit": 0.0,
+        "daily_return_pct": 0.0,
+        "cumulative_profit": 0.0,
+        "cumulative_return_pct": 0.0,
+        "created_at": "2025-01-16T09:30:00Z"
+    }
+
+
+@pytest.fixture
+def mock_config():
+    """Mock configuration for testing."""
+    return {
+        "agent_type": "BaseAgent",
+        "date_range": {
+            "init_date": "2025-01-16",
+            "end_date": "2025-01-17"
+        },
+        "models": [
+            {
+                "name": "test-model",
+                "basemodel": "openai/gpt-4",
+                "signature": "test-model",
+                "enabled": True
+            }
+        ],
+        "agent_config": {
+            "max_steps": 10,
+            "max_retries": 3,
+            "base_delay": 0.5,
+            "initial_cash": 10000.0
+        },
+        "log_config": {
+            "log_path": "./data/agent_data"
+        }
+    }
+
+
+# Pytest configuration hooks
+def pytest_configure(config):
+    """Configure pytest with custom markers."""
+    config.addinivalue_line("markers", "unit: Unit tests (fast, isolated)")
+    config.addinivalue_line("markers", "integration: Integration tests (with dependencies)")
+    config.addinivalue_line("markers", "performance: Performance and benchmark tests")
+    config.addinivalue_line("markers", "security: Security tests")
+    config.addinivalue_line("markers", "e2e: End-to-end tests (Docker required)")
+    config.addinivalue_line("markers", "slow: Tests that take >10 seconds")