feat: export missing public API types for 1.0 surface

[rcbevans]

Summary

Exports eight types that users and third-party backend implementers already need but had to import from deep internal paths:

• Backend - the Protocol that storage backends implement
• Clock, SystemClock - time abstraction and its production implementation
• FakeClock - the test clock, now importable from the top level
• JobRow, JobPage - the data structures returned by job listing queries
• TaskQSettings, WorkerSettings - configuration types for the client and worker

Importing these from internal modules like taskq.backend._protocol or taskq.testing.clock would break on any future restructure. Adding them to __all__ makes them part of the stable public API surface for 1.0.

Test coverage

New tests/test_public_api_exports.py verifies:

• Each new export is importable from taskq and present in __all__
• Clock and Backend are @runtime_checkable Protocols
• SystemClock is a frozen dataclass with no fields
• FakeClock has now() and monotonic() methods
• JobRow is a frozen dataclass with id, status, and actor fields
• JobPage is a frozen dataclass with jobs and next_cursor fields
• TaskQSettings has a callable load classmethod
• WorkerSettings is a subclass of TaskQSettings
• Every name in __all__ is actually importable (regression guard)
• No duplicate entries in __all__
• Capitalised entries in __all__ are sorted

Verification

• ruff check - clean
• pyright - clean
• pytest tests/test_public_api_exports.py - all 16 tests pass