Add Jinja2 web UI with Bootstrap 5 dark theme

- app/templates/: base layout with sidebar nav + 6 pages
  (dashboard, printers, jobs, filaments, todos, pricing)
- app/routers/ui.py: HTML routes querying the DB live
- API routes moved to /api/* prefix
- requirements: jinja2, python-multipart

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

requirements.txt

@@ -4,3 +4,5 @@ sqlalchemy==2.0.36
 alembic==1.14.0
 pymysql==1.1.1
 cryptography==44.0.3
+jinja2==3.1.4
+python-multipart==0.0.20