Initial WhatToDo app

Self-hosted personal task list: FastAPI + SQLite in Docker, mobile-friendly
PWA frontend, deadlines, priorities, categories, tags, notes, checklists,
recurring tasks, single-password login, and JSON export/import.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docker-entrypoint.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+# Make sure the mounted data directory is writable, then drop root privileges.
+set -e
+mkdir -p "$(dirname "$DB_PATH")"
+chown -R app:app "$(dirname "$DB_PATH")"
+exec setpriv --reuid=app --regid=app --init-groups "$@"