chore: drop libpq-dev from Dockerfile (#118)

psycopg2-binary bundles its own libpq and does not need libpq-dev at
build time. curl is kept for the HEALTHCHECK.

Closes #118

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

Dockerfile

@@ -10,7 +10,6 @@ WORKDIR /usr/src/app
 
 # Install system dependencies (PostgreSQL client libraries)
 RUN apt-get update && apt-get install -y --no-install-recommends \
-    libpq-dev \
     curl \
     && rm -rf /var/lib/apt/lists/*