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>

templates/battery_delete.html

@@ -0,0 +1,29 @@
+{% extends "base.html" %}
+{% block title %}Delete {{ battery.label }} — Battery Tracker{% endblock %}
+
+{% block content %}
+<h1>Delete Battery</h1>
+
+<div class="card">
+  <p style="margin-bottom:1rem;">
+    You are about to <strong>permanently delete</strong> battery
+    <strong>{{ battery.label }}</strong> ({{ battery.brand }}).
+    This action cannot be undone.
+  </p>
+
+  {% if battery.is_installed() %}
+    <p class="text-warning" style="margin-bottom:1rem;">
+      ⚠ This battery is currently installed in
+      <strong>{{ battery.device.name }}</strong>.
+      Deleting it will free that slot.
+    </p>
+  {% endif %}
+
+  <div class="form-actions">
+    <form method="post" action="{{ url_for('battery_delete', battery_id=battery.id) }}">
+      <button class="btn btn-danger" type="submit">Confirm Delete</button>
+    </form>
+    <a class="btn btn-secondary" href="{{ url_for('battery_detail', battery_id=battery.id) }}">Cancel</a>
+  </div>
+</div>
+{% endblock %}