refactor(setup/pgsql): improve db setup credential handling and code organization

[joshtrichards]

Summary

Original code:

• Had weird coupling for $this->dbUser / $this->dbPassword that made logic harder to follow, troubleshoot, or change without introducing problems (e.g., userExists() implicitly checked whatever was in $this->dbUser)
• Used same code path for manual (pre-created by admin) credentials versus automatically generated credential mode
• Didn't use any helpers making it harder to read/isolate

Improvements:

1. Credential Management
   • Admin credentials now saved in local variables before mutation
   • Credentials generated entirely inside createDBUser() and only assigned to $this-> after success
   • userExists() now accepts explicit "dbUser" parameter
2. Code Organization
   • Separates automatic vs. manual setup paths (new performAutomaticSetup() introduced + less conditional logic depth)
   • Extracted key bits of existing logic into new helpers: checkCanCreateRoles(), verifyDatabaseConnection(), grantCreateOnPublicSchema()
   • Single $connection object/variable; less confusion between connection scenarios and more explicit lifecycle management
3. Observability
   • Better error context (user, database, exception details)
   • Some additional logging scenarios and clarity (e.g. when alternate username chosen)
   • Added 'app' => 'pgsql.setup' to all log contexts

In short:

• Clearer separation of concerns and elimination of hidden state dependencies. Behavior preserved; credentials saved to config remain identical.
• 172 lines -> 284 lines, but significantly more maintainable

Easiest to review probably by commit

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)