feat: scaffold Flask + React app with schema and JSON error handling

Claude-Session: https://claude.ai/code/session_01M9MLit5Ko3X4s7rzC5Kv7X

backend/auth.py

@@ -0,0 +1,8 @@
+"""Auth slice: password hashing (ADR-0001), server-side sessions (ADR-0002), and auth routes."""
+from flask import Blueprint, Flask
+
+bp = Blueprint("auth", __name__, url_prefix="/api/auth")
+
+
+def init_app(app: Flask) -> None:
+    app.register_blueprint(bp)