Add optional battery metadata fields

New nullable columns on Battery: size, chemistry, capacity_mah,
tested_capacity_mah, tested_date, charge_cycles, purchase_date.

Battery detail page shows all populated fields and a full edit form
with select dropdowns for size and chemistry (with Other fallback).
Capacity health % shown in green/orange/red when both nominal and
tested capacity are set. Dashboard gains a Size column.

models.py

@@ -37,6 +37,15 @@ class Battery(Base):
     device_id = Column(Integer, ForeignKey("device.id", ondelete="SET NULL"), nullable=True)
     notes = Column(Text, nullable=True)
 
+    # Optional metadata
+    size                = Column(String(20),  nullable=True)   # AA, AAA, 18650, CR2032 …
+    chemistry           = Column(String(20),  nullable=True)   # NiMH, Alkaline, Li-ion …
+    capacity_mah        = Column(Integer,     nullable=True)   # nominal capacity in mAh
+    tested_capacity_mah = Column(Integer,     nullable=True)   # last measured capacity in mAh
+    tested_date         = Column(String(10),  nullable=True)   # YYYY-MM-DD of last test
+    charge_cycles       = Column(Integer,     nullable=True)   # number of charge cycles
+    purchase_date       = Column(String(10),  nullable=True)   # YYYY-MM-DD when purchased
+
     device = relationship("Device", back_populates="batteries")
 
     def is_available(self):