feat: implement OmegaConf-based configuration management with uv

Configuration changes:
- Replace pydantic-settings with OmegaConf for flexible config management
- Support YAML config files via CONFIG_FILE environment variable
- Support environment variables with STEWARD__SECTION__KEY format
- Add config_schema.yaml as default configuration schema
- Create pydantic models for each configuration section for type safety
- Maintain backward compatibility via properties on Settings class
- Add CONFIGURATION.md with comprehensive setup and usage guide
- Update pyproject.toml to include config_schema.yaml in package data

Build/deployment changes:
- Replace pip with uv in Dockerfile for faster dependency installation
- Create docker-compose.dev.yml for local development (build .)
- Keep docker-compose.yml for production (uses ghcr.io/djw4/steward:latest)
- Update .env.example with new STEWARD__* variable format

This setup is designed for Kubernetes deployment:
- Non-sensitive config goes in ConfigMap (config.yaml)
- Secrets go in Secret resources (environment variables)
- Single unified configuration system for all environments

pyproject.toml

@@ -16,6 +16,7 @@ dependencies = [
     "apscheduler>=3.10",
     "pydantic>=2.7",
     "pydantic-settings>=2.3",
+    "omegaconf>=2.3",
 ]
 
 [project.optional-dependencies]
@@ -35,6 +36,9 @@ steward = "steward.main:main"
 where = ["."]
 include = ["steward*"]
 
+[tool.setuptools.package-data]
+steward = ["config_schema.yaml"]
+
 [tool.ruff]
 line-length = 100
 target-version = "py312"