feat(db): T06 create database connection and session management

- Add database.py with SQLAlchemy engine and session
- Implement get_db() for FastAPI dependency injection
- Implement init_db() for table creation
- Use SQLAlchemy 2.0 declarative_base() syntax
- Add comprehensive tests with 100% coverage

Tests: 11 passed, 100% coverage

src/openrouter_monitor/database.py

@@ -0,0 +1,67 @@
+"""Database connection and session management.
+
+T06: Database connection & session management
+"""
+from sqlalchemy import create_engine
+from sqlalchemy.orm import sessionmaker, Session, declarative_base
+from typing import Generator
+
+from openrouter_monitor.config import get_settings
+
+
+# Create declarative base for models (SQLAlchemy 2.0 style)
+Base = declarative_base()
+
+# Get settings
+settings = get_settings()
+
+# Create engine with SQLite configuration
+# check_same_thread=False is required for SQLite with async/threads
+engine = create_engine(
+    settings.database_url,
+    connect_args={"check_same_thread": False}
+)
+
+# Create session maker with expire_on_commit=False
+# This prevents attributes from being expired after commit
+SessionLocal = sessionmaker(
+    autocommit=False,
+    autoflush=False,
+    bind=engine,
+    expire_on_commit=False
+)
+
+
+def get_db() -> Generator[Session, None, None]:
+    """Get database session for FastAPI dependency injection.
+    
+    This function creates a new database session and yields it.
+    The session is automatically closed when the request is done.
+    
+    Yields:
+        Session: SQLAlchemy database session
+        
+    Example:
+        >>> from fastapi import Depends
+        >>> @app.get("/items/")
+        >>> def read_items(db: Session = Depends(get_db)):
+        ...     return db.query(Item).all()
+    """
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()
+
+
+def init_db() -> None:
+    """Initialize database by creating all tables.
+    
+    This function creates all tables registered with Base.metadata.
+    Should be called at application startup.
+    
+    Example:
+        >>> from openrouter_monitor.database import init_db
+        >>> init_db()  # Creates all tables
+    """
+    Base.metadata.create_all(bind=engine)