feat: database setup with SQLAlchemy ORM models for Device and DeviceLog

- Replace minimal database.py with full SQLAlchemy async setup
- Create Device and DeviceLog models with proper relationships
- Add tests for model creation and relationship functionality
- Fixed lazy loading issue in tests by avoiding .count() on dynamic relationships

src/mqtt_home/models.py

@@ -0,0 +1,49 @@
+import uuid
+from datetime import datetime, timezone
+
+from sqlalchemy import Column, String, Boolean, DateTime, Text, Integer, ForeignKey
+from sqlalchemy.orm import relationship
+
+from mqtt_home.database import Base
+
+
+def _utcnow():
+    return datetime.now(timezone.utc)
+
+
+def _new_id():
+    return str(uuid.uuid4())
+
+
+class Device(Base):
+    __tablename__ = "devices"
+
+    id = Column(String(36), primary_key=True, default=_new_id)
+    name = Column(String(200), nullable=False)
+    type = Column(String(50), nullable=False, default="switch")
+    protocol = Column(String(20), nullable=False, default="custom")
+    mqtt_topic = Column(String(500), nullable=False)
+    command_topic = Column(String(500), nullable=True)
+    discovery_topic = Column(String(500), nullable=True)
+    discovery_payload = Column(Text, nullable=True)
+    attributes = Column(Text, nullable=True, default="{}")
+    state = Column(String(500), nullable=True, default=None)
+    is_online = Column(Boolean, nullable=False, default=False)
+    last_seen = Column(DateTime(timezone=True), nullable=True)
+    created_at = Column(DateTime(timezone=True), nullable=False, default=_utcnow)
+    updated_at = Column(DateTime(timezone=True), nullable=False, default=_utcnow, onupdate=_utcnow)
+
+    logs = relationship("DeviceLog", back_populates="device", lazy="select")
+
+
+class DeviceLog(Base):
+    __tablename__ = "device_logs"
+
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    device_id = Column(String(36), ForeignKey("devices.id", ondelete="CASCADE"), nullable=False)
+    direction = Column(String(10), nullable=False)
+    topic = Column(String(500), nullable=False)
+    payload = Column(Text, nullable=False)
+    timestamp = Column(DateTime(timezone=True), nullable=False, default=_utcnow)
+
+    device = relationship("Device", back_populates="logs")