Integrate release-plz for automated releases

[rubenhensen]

Summary

• Add release-plz workflow and .release-plz.toml to automate version bump PRs and GitHub release creation per crate
• Update delivery.yml to use crate-prefixed tags (pg-pkg-v* for Docker, pg-wasm-v* for npm) instead of plain v* tags
• Skip Docker build/push jobs for wasm-only releases

Prerequisites

Before merging, the following setup is needed:

1. Create a GitHub App with Contents: write permission for this repo
2. Configure RELEASE_APP_ID as a repo variable and RELEASE_APP_PRIVATE_KEY as a repo secret
3. Push bootstrap tags for current versions so release-plz doesn't re-release unchanged crates:

   git tag pg-core-v0.3.0
   git tag pg-cli-v0.3.0-rc.0
   git tag pg-pkg-v0.3.0-rc.0
   git tag pg-wasm-v0.5.3
   git push origin --tags
   

See release-plz GitHub token docs for GitHub App setup instructions.

Test plan

• Verify CI passes on this PR (Docker builds still run for non-release pushes)
• After merging + setup: push to main creates/updates a release-plz PR
• Merging a release PR creates GitHub releases with correct <crate>-v<version> tags
• pg-pkg-v* release triggers Docker build with release profile and correct version tags
• pg-wasm-v* release triggers npm publish with correct version
• Non-release pushes to main still build edge Docker images