Simplify battery management: bulk add, device-level auto-install, mass operations

- Replace single-battery add form with bulk add (brand + count, auto-generated labels)
- Add device-level install form: specify brand+qty pairs, system autoselects available batteries
- Add bulk actions on dashboard: retire, delete, unassign, change brand (checkbox multi-select)
- Keep per-battery assign route for special cases (e.g. known low-capacity battery)
- Remove unique constraint on Battery.label (labels are now auto-generated)
- Add *.snapshot to .gitignore for DB snapshot files
- Rewrite tests: 35 passing (11 new tests for bulk-add, device-install, bulk-actions)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

models.py

@@ -31,7 +31,7 @@ class Battery(Base):
     __tablename__ = "battery"
 
     id = Column(Integer, primary_key=True, autoincrement=True)
-    label = Column(String(50), nullable=False, unique=True)
+    label = Column(String(50), nullable=False)
     brand = Column(String(100), nullable=False)
     status = Column(String(20), nullable=False, default="available")
     device_id = Column(Integer, ForeignKey("device.id", ondelete="SET NULL"), nullable=True)