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/api/root.py

@@ -1,5 +1,7 @@
 # retoor <retoor@molodetz.nl>
 
+import re
+
 import devplacepy.main as m
 from starlette.testclient import TestClient
 def _patch_limits(monkeypatch, limit, window=60):
@@ -43,3 +45,29 @@ def test_rate_limit_reads_configured_setting(monkeypatch):
     second = client.post("/", headers={"X-Real-IP": "7.7.7.7"}).status_code
     assert first != 429
     assert second == 429
+
+
+def test_response_time_header_present():
+    client = TestClient(m.app)
+    response = client.get("/")
+    assert response.status_code == 200
+    value = response.headers.get("X-Response-Time")
+    assert value is not None and value.endswith("ms")
+    assert float(value[:-2]) >= 0
+
+
+def test_response_time_indicator_rendered_on_html():
+    client = TestClient(m.app)
+    match = re.search(
+        r'<div class="response-time-indicator"[^>]*>([\d.]+) ms</div>',
+        client.get("/").text,
+    )
+    assert match is not None
+    assert float(match.group(1)) >= 0
+
+
+def test_response_time_indicator_absent_from_json():
+    client = TestClient(m.app)
+    response = client.get("/feed", headers={"Accept": "application/json"})
+    assert "response-time-indicator" not in response.text
+    assert response.headers.get("X-Response-Time", "").endswith("ms")