Three features: device dropdown filter, charge log history, unassign-all

- Device dropdowns (quick-assign, bulk install, assign page) now only show
  devices with free slots; full devices are excluded entirely
- New ChargeLog model tracks charge dates with optional cycle increment;
  battery detail page gets a Charge History card with add/delete rows
- Device list page gets per-device Unassign All button (with confirmation)
  via new POST /device/<id>/unassign-all route

models.py

@@ -54,6 +54,11 @@ class Battery(Base):
         order_by="CapacityTest.tested_date",
         cascade="all, delete-orphan",
     )
+    charge_logs = relationship(
+        "ChargeLog", back_populates="battery",
+        order_by="ChargeLog.charged_date",
+        cascade="all, delete-orphan",
+    )
 
     def is_available(self):
         return self.status == "available"
@@ -81,3 +86,18 @@ class CapacityTest(Base):
 
     def __repr__(self):
         return f"<CapacityTest {self.battery_id} {self.tested_date} {self.tested_capacity_mah}mAh>"
+
+
+class ChargeLog(Base):
+    __tablename__ = "charge_log"
+
+    id               = Column(Integer, primary_key=True, autoincrement=True)
+    battery_id       = Column(Integer, ForeignKey("battery.id", ondelete="CASCADE"), nullable=False)
+    charged_date     = Column(String(10), nullable=False)   # YYYY-MM-DD
+    increment_cycles = Column(Integer, nullable=False, default=0)  # 0 or 1
+    notes            = Column(Text, nullable=True)
+
+    battery = relationship("Battery", back_populates="charge_logs")
+
+    def __repr__(self):
+        return f"<ChargeLog {self.battery_id} {self.charged_date}>"