Fixed Docker build issues to complete UV migration.
## Changes
### README.md
- Created backend README.md (required by hatchling build system)
- Simple quick start guide referencing CLAUDE.md
### Dockerfile
- Added `build-essential` for compiling native extensions (pendulum Rust code)
- Updated copy steps to include README.md in both dev and prod stages
- Dockerfile now successfully builds both development and production images
### .dockerignore
- Added exception `!README.md` to allow README.md through
- Keeps other *.md files excluded as intended
## Testing
- ✅ Development image builds successfully (paper-dynasty-backend:dev-uv)
- ✅ Production image builds successfully (paper-dynasty-backend:prod-uv)
- ✅ Container starts and UV installs dependencies correctly
- ✅ Application attempts to start (fails only on missing .env, as expected)
## Build Results
- Dev image: 73 packages installed (with dev deps)
- Prod image: 57 packages installed (no dev deps)
- Both stages use `uv sync --frozen` for reproducible builds
- Build time: ~1-2 minutes per stage
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>