Initial commit: Flask battery tracker app

- Flask + SQLAlchemy (MariaDB-compatible schema) battery tracking web app
- 40 pre-seeded batteries (Eneloop, BONAI, Energizer NiMH) across 5 devices
- Business rules: block retired assignment, brand-mix warnings, capacity checks
- Mobile-friendly Jinja2 templates with inline CSS
- waitress WSGI server via systemd user service (sbin/install-service.sh)
- SQLite → MariaDB migration script (migrate_to_mariadb.py)
- 26 passing acceptance tests (pytest + Flask test client)

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

tests/conftest.py

@@ -0,0 +1,41 @@
+import os
+import tempfile
+
+import pytest
+
+
+@pytest.fixture()
+def app():
+    """Create app with a temporary file-based SQLite DB, removed after the test."""
+    db_fd, db_path = tempfile.mkstemp(suffix=".db")
+    os.close(db_fd)
+
+    class TestConfig:
+        SQLALCHEMY_DATABASE_URI = f"sqlite:///{db_path}"
+        SECRET_KEY = "test-secret"
+        SQLALCHEMY_TRACK_MODIFICATIONS = False
+        TESTING = True
+
+    from app import create_app
+    flask_app = create_app(TestConfig)
+
+    yield flask_app
+
+    os.unlink(db_path)
+
+
+@pytest.fixture()
+def client(app):
+    return app.test_client()
+
+
+@pytest.fixture()
+def seeded_client(app):
+    """Test client pre-loaded with 2 devices and 3 batteries (2 available, 1 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"})
+        yield c