docs: fix facts in docs and the missing-client error message

[lesnik512]

Summary

Several statements in the docs and README did not match the code, and one error message gave advice the broker cannot follow.

Changes

• docs/introduction/installation.md: "Python 3.13+" changed to 3.11+, matching requires-python = ">=3.11,<4" in pyproject.toml.
• Worst-case idle latency was documented as max_polling_interval × 1.5. subscriber/schedule.py:50 computes min(base * 2**n * jitter, max_idle), so jitter is applied before the cap and the worst case is max_polling_interval. Fixed in how-it-works.md and subscriber.md; the jitter mentions in README.md and the subscriber.md table now say it is applied before the cap.
• docs/usage/basic.md: timeline_key / payloads_key are prefixes; TimerStore._keys (store.py:95-97) builds {key}:{topic}.
• how-it-works.md pointed to the broker docstring for the Cluster restriction, but that docstring does not mention it. The check is broker.py:142, which raises TypeError; the docs now say so.
• subscriber.md: removed "NoCast-typed"; the example uses StreamMessage[TimerMessage].
• publisher.md: removed an unused StreamMessage import.
• router.md: async def handle_invoice(...): ... was not valid Python; it now byte-compiles.
• configs.py:28: "Connection not available. Connect the broker first." The client only comes from the constructor (broker.py:150, ConnectionState(client)); connect() cannot supply one. New message: "TimersBroker has no Redis client; pass one to TimersBroker(client)." The existing test now asserts the message; it failed before the change.
• Plainer wording in basic.md for timer ID overwrites and cancel_all side effects.

All Python blocks in README.md and docs/ byte-compile, ruff reports no unused imports in them, and the three testing.md examples pass under pytest.

Checklist

• Lint and format pass (ruff)
• Type check passes (ty)
• Tests pass and new behavior is covered
• Build succeeds (uv build) if packaging or build config changed
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging