feat(reminders): add generic webhook as a fourth reminder channel (#2952)

Replaces any Discord-specific reminder channel with a generic outbound
webhook channel. Users pick any saved Integration as the target and
supply a JSON payload template with {{title}} and {{message}}
placeholders — values are JSON-escaped before substitution. Works with
Discord, Slack, Teams, ntfy (JSON mode), or any service that accepts a
POST with a JSON body.

- `src/settings.py` — reminder_webhook_integration_id +
  reminder_webhook_payload_template defaults
- `routes/note_routes.py` — webhook delivery block; Integration lookup,
  template rendering, auth wiring; built-in preset defaults so
  discord_webhook works out of the box without a configured template;
  settings_override kwarg avoids test-button race condition
- `routes/auth_routes.py` — discord_webhook preset test handler
- `src/integrations.py` — discord_webhook preset with description +
  example templates; hides auth/key fields in the Integration form
- `src/builtin_actions.py` — webhook_sent delivery check
- `src/tool_implementations.py` — webhook aliases + enum updated
- `static/index.html` — Webhook channel option; Integration picker +
  payload template textarea
- `static/js/settings.js` — Integration list, populateWebhookIntegrations,
  syncChannelRows, hints, load/save, auto-fill preset templates,
  test-button override payload, hide auth/key for URL-auth presets

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

static/index.html

@@ -1957,6 +1957,7 @@
                   <option value="browser">Browser notification (default)</option>
                   <option value="email" id="set-reminder-channel-email-opt">Email</option>
                   <option value="ntfy" id="set-reminder-channel-ntfy-opt">ntfy</option>
+                  <option value="webhook" id="set-reminder-channel-webhook-opt">Webhook</option>
                 </select>
               </div>
               <div id="set-reminder-email-from-row" class="settings-row" style="display:none">
@@ -1971,13 +1972,21 @@
                 <label class="settings-label">ntfy topic</label>
                 <input id="set-reminder-ntfy-topic" class="settings-select" type="text" placeholder="reminders" />
               </div>
+              <div id="set-reminder-webhook-intg-row" class="settings-row" style="display:none">
+                <label class="settings-label">Integration</label>
+                <select id="set-reminder-webhook-intg" class="settings-select"></select>
+              </div>
+              <div id="set-reminder-webhook-template-row" class="settings-row" style="display:none;align-items:flex-start">
+                <label class="settings-label" style="padding-top:6px">Payload</label>
+                <textarea id="set-reminder-webhook-template" class="settings-select" rows="3" style="font-family:inherit;resize:vertical;flex:1" placeholder='{"content": "{{title}}: {{message}}"}'></textarea>
+              </div>
               <div id="set-reminder-channel-hint" style="font-size:11px;opacity:0.6;"></div>
               <div style="font-size:11px;opacity:0.6;margin-top:4px;">Configure email account, ntfy server, etc. in <a href="#" id="set-reminders-open-integrations" style="color:var(--accent, var(--red));text-decoration:none;font-weight:600;">Integrations</a>.</div>
             </div>
           </div>
           <div class="admin-card">
             <h2 style="display:flex;align-items:center;gap:6px;"><svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" style="margin-right:1px;opacity:0.6;flex-shrink:0"><path d="M12 0L14.59 8.41L23 12L14.59 15.59L12 24L9.41 15.59L1 12L9.41 8.41Z"/></svg>AI Synthesis<span style="flex:1"></span><label class="admin-switch" title="Use the utility model to write reminder messages"><input type="checkbox" id="set-reminder-llm-toggle"><span class="admin-slider"></span></label></h2>
-            <div class="admin-toggle-sub" style="margin-bottom:8px">When on, the utility model writes a short, warm one-line reminder for browser, email, AND ntfy reminders instead of just the raw note content.</div>
+            <div class="admin-toggle-sub" style="margin-bottom:8px">When on, the utility model writes a short, warm one-line reminder for browser, email, ntfy, AND webhook reminders instead of just the raw note content.</div>
           </div>
           <div class="admin-card">
             <h2><svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" style="vertical-align:-2px;margin-right:5px;opacity:0.6"><path d="M10 13a5 5 0 0 0 7.54.54l3-3a5 5 0 0 0-7.07-7.07l-1.72 1.71"/><path d="M14 11a5 5 0 0 0-7.54-.54l-3 3a5 5 0 0 0 7.07 7.07l1.71-1.71"/></svg>Public App URL</h2>