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/auth.py

@@ -2,7 +2,7 @@ import uuid
 
 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 hash_password, verify_password, get_current_user
@@ -12,7 +12,6 @@ from web.models import User
 from web.schemas import validate_registration, validate_login
 
 router = APIRouter()
-templates = Jinja2Templates(directory="web/templates")
 
 
 @router.get("/register")