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>

tests/conftest.py

@@ -31,11 +31,18 @@ def client(app):
 
 @pytest.fixture()
 def seeded_client(app):
-    """Test client pre-loaded with 2 devices and 3 batteries (2 available, 1 retired)."""
+    """Test client pre-loaded with 2 devices and 3 batteries (2 available, 1 retired).
+
+    Batteries:
+      id=1  BrandX 001  (BrandX, available)
+      id=2  BrandY 001  (BrandY, available)
+      id=3  BrandX 002  (BrandX, retired)
+    """
     with app.test_client() as c:
         c.post("/device/add", data={"name": "Device A", "battery_slots": "2"})
         c.post("/device/add", data={"name": "Device B", "battery_slots": "1"})
-        c.post("/battery/add", data={"label": "TST-01", "brand": "BrandX", "status": "available"})
-        c.post("/battery/add", data={"label": "TST-02", "brand": "BrandY", "status": "available"})
-        c.post("/battery/add", data={"label": "TST-03", "brand": "BrandX", "status": "retired"})
+        c.post("/battery/add", data={"brand": "BrandX", "count": "1"})  # id=1
+        c.post("/battery/add", data={"brand": "BrandY", "count": "1"})  # id=2
+        c.post("/battery/add", data={"brand": "BrandX", "count": "1"})  # id=3
+        c.post("/battery/3/retire")
         yield c