Add capacity test history and chart to battery detail

- New CapacityTest model (battery_id FK CASCADE, mah, date, notes)
- DB migration: create capacity_test table, migrate existing single-test data
- Two new routes: add and delete capacity test records
- Battery.tested_capacity_mah/tested_date kept in sync with latest test
  so dashboard display requires no changes
- Battery detail: Capacity History card with sortable table, health %
  per reading, and a canvas line chart (shown when >= 2 records)
- Chart uses CSS variables for colors — works in light and dark mode
- Remove tested_capacity_mah/tested_date from Edit Details form
- 3 new acceptance tests (48 total)

models.py

@@ -49,6 +49,11 @@ class Battery(Base):
     storage_location    = Column(String(100), nullable=True)   # where stored when not installed
 
     device = relationship("Device", back_populates="batteries")
+    capacity_tests = relationship(
+        "CapacityTest", back_populates="battery",
+        order_by="CapacityTest.tested_date",
+        cascade="all, delete-orphan",
+    )
 
     def is_available(self):
         return self.status == "available"
@@ -61,3 +66,18 @@ class Battery(Base):
 
     def __repr__(self):
         return f"<Battery {self.label}>"
+
+
+class CapacityTest(Base):
+    __tablename__ = "capacity_test"
+
+    id                  = Column(Integer, primary_key=True, autoincrement=True)
+    battery_id          = Column(Integer, ForeignKey("battery.id", ondelete="CASCADE"), nullable=False)
+    tested_capacity_mah = Column(Integer, nullable=False)
+    tested_date         = Column(String(10), nullable=False)  # YYYY-MM-DD
+    notes               = Column(Text, nullable=True)
+
+    battery = relationship("Battery", back_populates="capacity_tests")
+
+    def __repr__(self):
+        return f"<CapacityTest {self.battery_id} {self.tested_date} {self.tested_capacity_mah}mAh>"