Add release checklist for Pi and Docker validation

RELEASE_CHECKLIST.md

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+# Stegasoo Release Checklist
+
+Pre-release validation checklist. Complete all items before tagging a release.
+
+## Code Quality
+
+- [ ] All tests pass: `./venv/bin/pytest tests/ -v`
+- [ ] No lint errors: `./venv/bin/ruff check src/`
+- [ ] Version bumped in `pyproject.toml`
+- [ ] CHANGELOG.md updated
+
+## Pi Image Validation
+
+- [ ] Fresh Pi OS install with setup.sh works
+- [ ] First-boot wizard completes successfully
+- [ ] MOTD shows correct URL on SSH login
+- [ ] Smoke test passes: `./rpi/smoke-test.sh --443 <PI_IP>`
+- [ ] Encode/decode works on large image (10MB+)
+- [ ] Sanitize script runs cleanly
+- [ ] Image created and compressed
+
+## Docker Validation
+
+- [ ] Base image builds: `docker build -f Dockerfile.base -t stegasoo-base:latest .`
+- [ ] Web image builds: `docker-compose build web`
+- [ ] Container starts: `docker-compose up -d web`
+- [ ] Web UI accessible at http://localhost:5000
+- [ ] Encode/decode works in container
+- [ ] Container stops cleanly: `docker-compose down`
+
+## Release Process
+
+- [ ] Merge feature branch to main
+- [ ] Create annotated tag: `git tag -a vX.Y.Z -m "message"`
+- [ ] Push tag: `git push origin vX.Y.Z`
+- [ ] Create GitHub Release with release notes
+- [ ] Upload Pi image (.img.zst.zip)
+- [ ] Verify download links work
+
+## Post-Release
+
+- [ ] Delete old/obsolete releases if needed
+- [ ] Update any external documentation
+- [ ] Announce release (if applicable)