Initial commit: habit tracker PWA with Google OAuth, push notifications

FastAPI + SQLAlchemy/Alembic + MariaDB backend with Jinja2/htmx/Alpine
server-rendered frontend. Multi-user via Google OAuth, daily habit
tracking, monthly/weekly history views, Web Push reminders via
APScheduler, and PWA support (manifest, service worker, offline caching).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/templates/500.html

@@ -0,0 +1,11 @@
+{% extends "base.html" %}
+{% block title %}오류가 발생했어요 · 습관 트래커{% endblock %}
+{% block content %}
+<div style="min-height: 60vh; display:flex; align-items:center; justify-content:center; text-align:center;">
+  <div>
+    <h1>문제가 발생했어요</h1>
+    <p>일시적인 오류일 수 있어요. 잠시 후 다시 시도해주세요.</p>
+    <a href="/today" class="btn btn-primary">오늘 화면으로</a>
+  </div>
+</div>
+{% endblock %}