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/integrations.py

@@ -100,6 +100,19 @@ INTEGRATION_PRESETS: Dict[str, Dict[str, Any]] = {
             "  GET /{topic}/json?poll=1 — poll for messages"
         ),
     },
+    "discord_webhook": {
+        "name": "Discord Webhook",
+        "auth_type": "none",
+        "description": (
+            "Discord Incoming Webhook. Paste the full webhook URL (including the token) as the Base URL.\n"
+            "To get a URL: Discord server -> Server Settings -> Integrations -> Webhooks -> New Webhook -> Copy Webhook URL.\n"
+            "The secret is embedded in the URL — leave auth type as None.\n\n"
+            "Use this integration as the target in Settings -> Reminders -> Webhook channel.\n"
+            "Payload template examples:\n"
+            "  Simple:  {\"content\": \"{{title}}: {{message}}\"}\n"
+            "  Embed:   {\"embeds\": [{\"title\": \"{{title}}\", \"description\": \"{{message}}\", \"color\": 5793266}]}"
+        ),
+    },
     "vaultwarden": {
         "name": "Vaultwarden",
         "auth_type": "header",