fix(compose): mount db volume at /var/lib/postgresql for postgres 18

[obrien-k]

Summary

As merged (postgres 16-alpine → 18-alpine, #29), docker-compose.yml cannot start db at all — reproduced locally on both a fresh volume and one with existing data:

Error: in 18+, these Docker images are configured to store database data in a
       format which is compatible with "pg_ctlcluster" (specifically, using
       major-version-specific directory names)...

Postgres 18's images changed the expected volume convention: a single mount at /var/lib/postgresql, with data placed in a version-specific subdirectory the entrypoint manages itself, instead of a mount directly at .../data (docker-library/postgres#1259). api/ui never got a chance to start either, since they depend on db being healthy.

Change

Mount at /var/lib/postgresql instead of /var/lib/postgresql/data.

⚠️ Not an upgrade path

This makes 18 boot, nothing more. If anyone has real data sitting in ./volumes/db-data/ from a prior 16.x deployment, it's laid out directly in that directory (PG_VERSION, base/, etc.) — with this mount-path change, postgres 18 will look for data in a subdirectory of the same mount, not find the old files, and silently initialize a fresh empty database next to the orphaned 16 data. Nothing gets deleted, but it'll look like data loss. A real upgrade needs pg_upgrade (or dump/restore) run explicitly before/during this transition — this PR doesn't attempt that, it only fixes the mount so 18 can start at all (matches the state before #29, where nobody could get past db failing to start anyway).

Test plan

• Fresh volume: db reports healthy, 28 pending migrations apply cleanly, seed runs, api boots
• Confirmed end-to-end through nginx: curl http://localhost/api/version → 200 {"version":"0.7.0"}
• Verify no one has an existing populated 16.x volume depending on this repo's compose file before merging (see caveat above)

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

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