fix(db): respect schema_paths order in diff

[pjpjq]

Summary

• Preserve explicit [db.migrations].schema_paths order when preparing declared schemas for db diff.
• Expand directory schema path entries deterministically without overriding explicit file/glob order.
• Use pg-delta declarative apply only when schema_paths is unset or points directly at the declarative directory.

Fixes #3854

Testing

• go test ./internal/db/diff ./internal/db/declarative ./cmd