Fix dev setup Postgres migration detection

[wesbillman]

Summary

• load .env before running pgschema and fall back to the repo default Postgres env vars
• only retry migrations while Postgres itself is not yet accepting connections
• surface the real pgschema error output and print the resolved connection values in the success banner

Testing

• bash -n scripts/dev-setup.sh
• pre-commit: desktop-check, cargo fmt --check
• pre-push: desktop-check, desktop-build, cargo check --manifest-path desktop/src-tauri/Cargo.toml, cargo clippy --workspace --all-targets -- -D warnings, ./scripts/run-tests.sh unit