feat: auto-derive idempotency keys on @queue.task

[pratyush618]

Summary

Turns the existing unique_key deduplication infrastructure (already present
end-to-end in SQLite, Postgres, and Redis with partial unique indexes / Lua
scripts) into a first-class, opt-in task feature. No Rust changes, no schema
migration, no new Storage methods — pure-Python wiring.

@queue.task(idempotent=True)
def charge(customer_id: int, amount: int) -> str: ...

charge.delay(42, 1000)
charge.delay(42, 1000)   # → same job ID, no duplicate execution

• Auto-key format: auto:{sha256(task_name|serialized_payload)[:32]}. Hashed
  after the task's serializer runs, so the wire-payload contract is what
  decides equivalence.
• Per-call idempotency_key="…" overrides the auto-derived key.
• Per-call idempotent=False disables auto-derivation for one submission even
  on a task registered with idempotent=True.
• Existing unique_key parameter kept as a documented alias for back-compat.
• Mirrored on enqueue_many() via idempotency_keys: list[str | None] and a
  uniform idempotent flag.
• on_enqueue middleware sees and can mutate the new keys.

Slot release semantics are unchanged — the dedup window is held only while the
job is pending or running; once it terminates, a fresh call with the same
arguments creates a new job. This is enforced by the existing partial unique
index, not new code.

Files

• py_src/taskito/app.py — _task_idempotent map, _resolve_unique_key()
  helper, new kwargs on enqueue() / enqueue_many().
• py_src/taskito/mixins/decorators.py — idempotent: bool = False on
  @queue.task().
• py_src/taskito/task.py — TaskWrapper.apply_async() plumbs the new kwargs.
• tests/python/test_idempotent.py — 8 new tests.
• docs/content/docs/guides/reliability/idempotency.mdx + meta.json — guide.

Test plan

• uv run python -m pytest tests/python/test_idempotent.py tests/python/test_unique.py -v (12 passed)
• uv run python -m pytest tests/python/ -x (506 passed, 9 skipped)
• uv run ruff check py_src/ tests/python/test_idempotent.py
• uv run mypy py_src/taskito/ --no-incremental (clean across 121 files)
• cargo check --workspace (clean)
• CI green across SQLite / Postgres / Redis Rust jobs (no Rust touched, but worth confirming)