docs: document server-side rendering pipeline, response timing middleware, and Telegram pairing API

- Add comprehensive documentation for backend content rendering in AGENTS.md, detailing the new `render_content` and `render_title` Jinja globals built on mistune with media processing, emoji shortcodes, and XSS protection
- Document the `X-Response-Time` header and bottom-left render time indicator in README.md
- Update bot token pricing documentation to clarify fallback vs gateway cost headers
- Add `email_accounts` to soft-delete tables and `idx_users_role` composite index in database schema
- Implement `telegram_pairings` and `telegram_links` table creation with column migration and indexes
- Add `/profile/{username}/telegram` endpoint to docs API with request/unpair actions
- Register `TelegramService` in main.py lifespan and add `response_timing` middleware emitting `X-Response-Time` header
- Introduce `TelegramPairForm` model and `guard_public_host_sync` synchronous host validation function

devplacepy/templates/base.html

@@ -193,6 +193,9 @@
     </footer>
 {% endblock %}
 
+    {%- set _response_time = response_time_ms(request) -%}
+    {%- if _response_time %}<div class="response-time-indicator" title="Server response time">{{ _response_time }} ms</div>{% endif -%}
+
     {% if user %}
     <template id="comment-reply-template">
         {% set _comment_target_uid = "" %}{% set _comment_target_type = "" %}{% include "_comment_form.html" %}