Bundle FastAPI backend into Next.js Docker container

The Next.js app was proxying /api/* to the old Flask/FastAPI server
at legal-ai.nautilus.marcusgroup.org. When that server went down,
the Next.js app's API calls failed with 503.

Now both services run in the same container:
- FastAPI (uvicorn) on :8000 — the API backend
- Next.js (node) on :3000 — proxies /api/* to localhost:8000

Changes:
- Dockerfile: multi-stage build with Python 3.12 + Node.js
- next.config.ts: default proxy target is now 127.0.0.1:8000
- start.sh: launches uvicorn in background + node in foreground
- pyproject.toml: add fastapi + uvicorn as explicit deps

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

start.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+# Start FastAPI backend + Next.js frontend in the same container.
+# Uvicorn runs in the background; Node.js runs in the foreground
+# so that Docker gets its exit signal from the primary process.
+
+set -e
+
+echo "Starting FastAPI backend on :8000 ..."
+uvicorn web.app:app --host 127.0.0.1 --port 8000 --workers 1 &
+UVICORN_PID=$!
+
+echo "Starting Next.js frontend on :3000 ..."
+exec node server.js