Refactor Docket and Worker to use backend abstraction

Updates:
- Docket now creates and uses RedisBackend via `_backend` field
- All Redis operations go through backend protocol methods
- Removed 400+ lines of direct Redis interaction code
- Worker accesses backend through `docket._backend`
- Updated all tests to use `docket._backend.redis()` instead of `docket.redis()`
- Removed tests for internal methods that no longer exist

This completes the backend abstraction, making Docket and Worker fully
backend-agnostic.

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

Co-Authored-By: Claude <[email protected]>

Author: zzstoatzz