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>

routes/auth_routes.py

@@ -585,6 +585,27 @@ def setup_auth_routes(auth_manager: AuthManager) -> APIRouter:
                     hint = " If this is Docker Compose ntfy, set NTFY_BIND to that host/Tailscale IP and NTFY_BASE_URL to the same server URL in .env, then recreate ntfy."
                 return {"ok": False, "message": f"ntfy publish to {full_url} failed: {e}.{hint}"[:500]}
 
+        if preset == "discord_webhook":
+            import httpx
+            webhook_url = (integ.get("base_url") or "").strip()
+            if not webhook_url:
+                return {"ok": False, "message": "No webhook URL set — paste the full Discord webhook URL into the Base URL field."}
+            payload = {
+                "embeds": [{
+                    "title": "Odysseus connectivity test",
+                    "description": "If you see this, your Discord Webhook integration is wired up correctly.",
+                    "color": 5793266,
+                }]
+            }
+            try:
+                async with httpx.AsyncClient(timeout=8.0) as client:
+                    r = await client.post(webhook_url, json=payload)
+                if r.is_success:
+                    return {"ok": True, "message": "Test embed sent — check your Discord channel to confirm it arrived."}
+                return {"ok": False, "message": f"Discord returned HTTP {r.status_code}: {r.text[:200]}"}
+            except Exception as e:
+                return {"ok": False, "message": f"Request failed: {e}"[:400]}
+
         # All other presets: GET against a known health endpoint.
         # Fall back to detecting from name if preset is missing.
         health_paths = {