docs(scheduler): add comprehensive README and CHANGELOG

Add complete documentation for The Scheduler service.

README.md (500+ lines):
- Architecture overview with ASCII diagram
- Quick start guide
- Complete API reference with curl examples
- Task scheduling patterns and examples
- Priority system documentation
- Built-in executors documentation (example, doc_sync, config_backup)
- Custom executor development guide
- Current tasks table
- Database schema documentation
- Testing guide with coverage metrics
- Development and debugging information
- Monitoring and troubleshooting
- Security and performance notes
- API reference with response codes and filtering

CHANGELOG.md:
- Initial v1.0.0 release documentation
- Core features and architecture
- REST API endpoints
- Task executors and pre-configured tasks
- Testing infrastructure and metrics
- Technical details and dependencies
- Coverage metrics breakdown
- Planned features for future releases

Documentation covers:
- All API endpoints and authentication
- Scheduling examples (every minute, daily, monthly, etc.)
- Priority ranges and usage
- Executor configuration
- Test database setup
- Docker stack configuration
- Common issues and solutions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

services/scheduler/src/tasks/__init__.py

@@ -0,0 +1 @@
+"""Task execution system for The Scheduler."""