fix: optional Postgres via compose-with-db overlay

[zdrapela]

Description

Fixes RHDHBUGS-1865: enabling the optional PostgreSQL service no longer requires uncommenting blocks in the default compose.yaml (which caused merge conflicts on pull).

• Add compose-with-db.yaml overlay (same pattern as compose-with-corporate-proxy.yaml / compose-dynamic-plugins-root.yaml)
• Point compose.yaml at the overlay instead of a commented db service
• Update the PostgreSQL guide for -f compose.yaml -f compose-with-db.yaml, including a combined example with compose-with-corporate-proxy.yaml
• Point major-upgrade steps at compose-with-db.yaml and dual -f compose commands

Stacked on #275 (docs/pg16-to-pg18-upgrade). Retarget to main after #275 merges.

Which issue(s) does this PR fix or relate to

• Fixes RHDHBUGS-1865
• Depends on docs: PostgreSQL major upgrade for RHDH Local #275

PR acceptance criteria

• Tests updated and passing
• Documentation updated
• Built-in TechDocs updated if needed. Note that TechDocs changes may need to be reviewed by a Product Manager and/or Architect to ensure content accuracy, clarity, and alignment with user needs.

How to test changes / Special notes to the reviewer

1. Do not edit compose.yaml for Postgres
2. podman login registry.redhat.io
3. Copy POSTGRES_* into .env; switch app-config.local.yaml from SQLite to pg as in the guide
4. podman compose -f compose.yaml -f compose-with-db.yaml up -d → db healthy, RHDH up
5. Optional: podman compose -f compose.yaml -f compose-with-db.yaml -f compose-with-corporate-proxy.yaml config merges db + proxy + rhdh.depends_on
6. npx --yes dclint compose-with-db.yaml → 0 errors

Made with Cursor

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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