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/feed.py

@@ -871,12 +871,12 @@ def test_admin_has_admin_nav(alice):
     assert page.locator("a[href='/admin']").count() >= 1
 
 
-def test_admin_sees_role_badges(alice):
+def test_feed_shows_author_level(alice):
     page, _ = alice
     _seed_post_role_visibility()
     page.goto(f"{BASE_URL}/feed", wait_until="domcontentloaded")
     page.locator(".post-card").first.wait_for(state="visible")
-    assert page.locator(".post-author-role").count() >= 1
+    assert page.locator(".post-author-level").count() >= 1
 
 
 def test_feed_canonical_preserves_pagination(page, app_server):