fix(docker): create backend/docs directory before docs build

backend/docs/ is in .gitignore so it doesn't exist in the Docker build
context. The docs build script copies output to ../backend/docs/ but
fails silently when the directory is missing, resulting in /docs
returning 404 in the deployed container.

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

Author: pescn

Dockerfile

@@ -12,6 +12,7 @@ RUN --mount=type=cache,target=/cache \
     bun ci
 
 COPY . .
+RUN mkdir -p backend/docs
 RUN bun run build
 RUN cd docs && bun run build