fix(svc-db-postgres): pin Postgres version to 17-3.5, add entity_name var, and dynamically resolve major version for dev package

- Changed default Docker image version from 'latest' to '17-3.5' in config
- Introduced entity_name var for consistent lookups
- Added POSTGRES_VERSION and POSTGRES_VERSION_MAJOR extraction
- Updated Dockerfile to install postgresql-server-dev-<major> with default fallback to 'all'
- Minor YAML formatting improvements

Ref: https://chatgpt.com/share/68de40b4-2eb8-800f-ab5b-11cc873c3604

roles/svc-db-postgres/templates/Dockerfile.j2

@@ -5,7 +5,7 @@ RUN apt-get update \
  && apt-get install -y --no-install-recommends \
       build-essential \
       git \
-      postgresql-server-dev-all \
+      postgresql-server-dev-{{ POSTGRES_VERSION_MAJOR | default('all', true) }} \
  && git clone https://github.com/pgvector/pgvector.git /tmp/pgvector \
  && cd /tmp/pgvector \
  && make \