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>

src/settings.py

@@ -141,10 +141,17 @@ DEFAULT_SETTINGS = {
     # library can grow beyond this; cleanup/retirement is an explicit review flow.
     "skill_max_injected": 3,
     # Reminders
-    "reminder_channel": "browser",   # "browser" | "email" | "ntfy"
+    "reminder_channel": "browser",   # "browser" | "email" | "ntfy" | "webhook"
     "reminder_llm_synthesis": False,
     "reminder_ntfy_topic": "Reminders",
     "reminder_email_to": "",
+    # Generic outbound webhook channel: pick any saved Integration as the
+    # target and supply a JSON payload template. Use {{title}} and {{message}}
+    # as placeholders — they are JSON-escaped before substitution, so the
+    # rendered string is always valid JSON. Works with Discord, Slack, Teams,
+    # ntfy (JSON mode), or any service that accepts a POST with a JSON body.
+    "reminder_webhook_integration_id": "",
+    "reminder_webhook_payload_template": "",
     # Email triage scanner rules. Running/paused state and schedule live in
     # Tasks via the built-in `check_email_urgency` task.
     "urgent_email_prompt": (