Add API tokens and OAuth2 client support for external integrations

- Personal API tokens (model, user-settings UI, admin, management command,
  DRF auth class) for non-interactive API access from automations like n8n.
  Raw token shown once; only a SHA-256 hash is stored; last_used_at writes
  are throttled.
- OAuth2 authorization server via django-oauth-toolkit with authorization
  server metadata and optional, off-by-default Dynamic Client Registration
  (RFC 7591), so remote OAuth/MCP clients can authenticate and self-register.
- Tests for token auth, DCR gating and the management commands, plus
  .env.example and README documentation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/templates/users/fragments/user_settings.html

@@ -8,4 +8,8 @@
 <form hx-post="{% url 'user_settings' %}" hx-target="#generic-offcanvas" novalidate>
     {% crispy form %}
 </form>
+<div class="divider my-6"></div>
+<div id="api-token-settings">
+    {% include "users/fragments/api_tokens.html" %}
+</div>
 {% endblock %}