feat: add per-device custom Chrome production URL override

- Device model: add custom_chrome_url nullable column
- API: GET /api/wmt/config/<mac> returns device custom_chrome_url when set,
  falls back to WMTGlobalConfig.chrome_url for all other devices
- Web: device_edit POST saves/clears custom_chrome_url from form
- Template: wmt/device_form.html adds Chrome URL override section with
  clear status indicator (using global vs custom URL)
- DB: ALTER TABLE devices ADD COLUMN custom_chrome_url VARCHAR(500) applied

templates/wmt/device_form.html

@@ -72,6 +72,34 @@
             </select>
           </div>
 
+          <hr>
+          <h6 class="text-muted mb-1">Chrome Launch – Production URL Override</h6>
+          <p class="text-muted small mb-3">
+            Leave blank to use the global default URL configured in
+            <a href="{{ url_for('wmt_web.settings') }}" target="_blank">WMT Settings</a>.
+            Fill in only if this device needs a different production page.
+          </p>
+          <div class="mb-3">
+            <label class="form-label fw-semibold">Custom Production URL
+              <small class="text-muted fw-normal">(device-specific override)</small>
+            </label>
+            <input type="url" name="custom_chrome_url" class="form-control"
+                   value="{{ device.custom_chrome_url or '' if device else '' }}"
+                   placeholder="Leave blank to use global default">
+            {% if device and device.custom_chrome_url %}
+            <div class="form-text text-warning">
+              <i class="fas fa-exclamation-triangle me-1"></i>
+              This device uses a custom URL instead of the global default.
+              Clear the field to revert to the global setting.
+            </div>
+            {% else %}
+            <div class="form-text text-success">
+              <i class="fas fa-check-circle me-1"></i>
+              Using global default production URL.
+            </div>
+            {% endif %}
+          </div>
+
           {% if device %}
           <div class="alert alert-light border small mb-4">
             <strong>Last seen:</strong>