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/assign.html

@@ -0,0 +1,48 @@
+{% extends "base.html" %}
+{% block title %}Assign {{ battery.label }} — Battery Tracker{% endblock %}
+
+{% block content %}
+<h1>Assign {{ battery.label }}</h1>
+<p class="text-muted" style="margin-bottom:1rem;">Brand: {{ battery.brand }}</p>
+
+<div class="card">
+  {% if devices %}
+    <form method="post" action="{{ url_for('battery_assign', battery_id=battery.id) }}">
+      <div class="form-group">
+        <label>Select Device</label>
+        {% for device in devices %}
+          {% set full = device.installed_count() >= device.battery_slots %}
+          {% set mix  = device.installed_brands() and battery.brand not in device.installed_brands() %}
+          <div style="margin-bottom:0.75rem;padding:0.6rem 0.75rem;border:1px solid #e2e8f0;border-radius:4px;
+                      {% if full %}opacity:0.5;{% endif %}background:#fff;">
+            <label style="display:flex;align-items:center;gap:0.6rem;font-weight:normal;cursor:{% if full %}not-allowed{% else %}pointer{% endif %};">
+              <input type="radio" name="device_id" value="{{ device.id }}"
+                     {% if full %}disabled{% endif %}
+                     style="cursor:{% if full %}not-allowed{% else %}pointer{% endif %};">
+              <span>
+                <strong>{{ device.name }}</strong>
+                <span class="text-muted">({{ device.installed_count() }}/{{ device.battery_slots }} slots used)</span>
+                {% if full %}
+                  <span class="badge badge-retired">Full</span>
+                {% endif %}
+              </span>
+            </label>
+            {% if mix and not full %}
+              <p class="text-warning" style="margin-top:0.3rem;margin-left:1.6rem;">
+                ⚠ Already has {{ device.installed_brands()|join(', ') }} — mixing brands not recommended.
+              </p>
+            {% endif %}
+          </div>
+        {% endfor %}
+      </div>
+
+      <div class="form-actions">
+        <button class="btn btn-primary" type="submit">Assign Battery</button>
+        <a class="btn btn-secondary" href="{{ url_for('battery_detail', battery_id=battery.id) }}">Cancel</a>
+      </div>
+    </form>
+  {% else %}
+    <p class="text-muted">No devices exist yet. <a href="{{ url_for('device_add') }}">Add a device first.</a></p>
+  {% endif %}
+</div>
+{% endblock %}