Add Jivosite live chat widget support

Resolves #3 — widget is loaded on every page via base.html when
JIVOSITE_WIDGET_ID env var is set. Centralized Jinja2Templates instance
in web/templates_env.py with jivosite_widget_id as a global.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/routes/vk.py

@@ -5,7 +5,7 @@ import httpx
 
 from fastapi import APIRouter, Request, Depends
 from fastapi.responses import RedirectResponse
-from fastapi.templating import Jinja2Templates
+from web.templates_env import templates
 from sqlalchemy.orm import Session
 
 from web.auth import get_current_user
@@ -14,7 +14,6 @@ from web.database import get_db
 from web.models import User, VkConnection
 
 router = APIRouter(prefix="/vk")
-templates = Jinja2Templates(directory="web/templates")
 
 VK_AUTHORIZE_URL = "https://oauth.vk.com/authorize"
 VK_TOKEN_URL = "https://oauth.vk.com/access_token"