fix(docker): update postgres volume mount for v18 compatibility

[naraen-ram]

Purpose / Description

Fix PostgreSQL 18 container failing to start after the automated version bump in 0905772.

PostgreSQL 18+ changed the data directory layout — it expects the volume mount at /var/lib/postgresql (not /var/lib/postgresql/data). The renovate bot bumped postgres:16 → postgres:18 without updating the volume mount, breaking docker compose up for all users.

Fixes

• Fixes the breakage introduced by 0905772 (chore(deps): update postgres docker tag to v18)

Approach

Changed the volume mount from pgdata:/var/lib/postgresql/data to pgdata:/var/lib/postgresql in docker/docker-compose.yml. This matches the PostgreSQL 18+ requirement documented at docker-library/postgres#1259.

How Has This Been Tested?

• docker compose down -v && docker compose up -d — all 10 services start healthy
• PostgreSQL container passes healthcheck (pg_isready)
• Init container successfully runs migrations and seeds demo accounts
• API container starts and serves requests on port 8000

Checklist

• All commits are signed off (git commit -s) per the DCO
• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (N/A - docker config only)