Add optional Home Assistant integration for battery percentage tracking

models.py

@@ -12,6 +12,7 @@ class Device(Base):
     battery_slots = Column(Integer, nullable=False, default=1)
     device_type   = Column(String(50),  nullable=True)
     notes = Column(Text, nullable=True)
+    ha_entity_id = Column(String(100), nullable=True)   # e.g. "sensor.tv_remote_battery"
 
     batteries = relationship("Battery", back_populates="device")
 
@@ -47,6 +48,7 @@ class Battery(Base):
     charge_cycles       = Column(Integer,     nullable=True)   # number of charge cycles
     purchase_date       = Column(String(10),  nullable=True)   # YYYY-MM-DD when purchased
     storage_location    = Column(String(100), nullable=True)   # where stored when not installed
+    battery_percentage  = Column(Integer,     nullable=True)   # current charge %, set by HA poller or manually
 
     device = relationship("Device", back_populates="batteries")
     capacity_tests = relationship(