Fix prod startup: add start.sh for backend, fix documents proxy base route

- backend/Dockerfile: run migrations via start.sh before uvicorn instead
  of launching uvicorn directly (prod was skipping Alembic)
- backend/scripts/start.sh: alembic upgrade head + uvicorn exec
- documents_proxy.py: add explicit "" route so GET /api/documents (no
  trailing slash) returns 200 instead of 307 redirect
- README.md: update Containers table, volumes section, and Current State
  to reflect the new 4-container architecture with doc-service

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

backend/scripts/start.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+set -e
+
+echo "[start] running migrations..."
+alembic upgrade head
+
+echo "[start] starting uvicorn..."
+exec uvicorn app.main:app --host 0.0.0.0 --port 8000