feat: web UI + SQLite storage + FastAPI backend

- app.py: unified app – ntfy subscriber runs as background thread,
  FastAPI serves REST API (/api/messages) and static frontend
- static/index.html: terminal-style notification table with
  sortable columns, drag & drop column reorder, column visibility
  toggle (localStorage), text search, forwarded-only filter,
  auto-refresh every 30s
- All ntfy fields stored: id, received_at, ntfy_time, topic,
  title, message, priority, tags, forwarded
- /data volume for persistent SQLite DB
- Port 8000 exposed

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

stack.env.example

@@ -9,3 +9,4 @@ TARGET_URL=https://ntfy.albert-zangerl.com/wichtig
 
 LOG_LEVEL=INFO
 RECONNECT_DELAY=10
+PORT=8000