Pin PGDATA so postgres:18 works with legacy volume mount

[rdimitrov]

Summary

• postgres:18 changed its default PGDATA to /var/lib/postgresql/$PG_MAJOR/docker (Change PGDATA in 18+ to /var/lib/postgresql/MAJOR/docker docker-library/postgres#1259). The entrypoint now refuses to start when it sees a mount at the legacy /var/lib/postgresql/data path — even on a fresh volume — exiting with unused mount/volume.
• Set PGDATA=/var/lib/postgresql/data explicitly on the postgres service in both docker-compose.yaml and docker-compose.smoke-test.yaml so the existing volume layout keeps working.

Why this minimal override

Upstream's recommended config for 18+ is to mount at /var/lib/postgresql (parent dir) so pg_upgrade --link works without crossing mount boundaries. That's a bigger change to the volume layout and would lose existing data on upgrade. Pinning PGDATA is the smallest diff that unbreaks docker compose up today; the parent-mount migration can be a follow-up if/when major-version upgrades are on the table.

Test plan

• docker compose down -v && docker compose up from a clean state — postgres reaches healthy, registry-api runs migrations and serves on :8080.
• docker compose -f docker-compose.smoke-test.yaml --project-name thv-smoke-test up --detach --wait succeeds (or run the smoke-test skill end-to-end).

Fixes #773

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.07%. Comparing base (8649e14) to head (e03b48d).

Additional details and impacted files

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##             main     #777   +/-   ##
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  Coverage   61.07%   61.07%           
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  Files         108      108           
  Lines       10593    10593           
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  Hits         6470     6470           
  Misses       3546     3546           
  Partials      577      577           

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