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/Dockerfile

@@ -24,9 +24,11 @@ COPY --from=builder /install /usr/local
 COPY --chown=appuser:appuser app ./app
 COPY --chown=appuser:appuser alembic ./alembic
 COPY --chown=appuser:appuser alembic.ini .
+COPY --chown=appuser:appuser scripts ./scripts
+RUN chmod +x scripts/start.sh
 
 USER appuser
 
 EXPOSE 8000
 
-CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]
+CMD ["sh", "scripts/start.sh"]