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

tests/e2e/post.py

@@ -328,7 +328,7 @@ def test_attachment_upload_ui(alice):
     file_input.set_input_files(
         {"name": "pic.png", "mimeType": "image/png", "buffer": buf.getvalue()}
     )
-    page.locator(".comment-form dp-upload .dp-upload-chip").first.wait_for(
+    page.locator(".comment-form dp-upload .dp-upload-count").first.wait_for(
         state="visible", timeout=15000
     )
     expect(