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>

config.py

@@ -0,0 +1,8 @@
+import os
+
+SQLALCHEMY_DATABASE_URI = os.environ.get(
+    "DATABASE_URL",
+    "sqlite:///batteries.db",
+)
+SECRET_KEY = os.environ.get("SECRET_KEY", "dev-secret-change-in-prod")
+SQLALCHEMY_TRACK_MODIFICATIONS = False