feat: add RedisStore for production multi-instance deployments

[0xzoz]

The existing MemoryStore works well for single-process deployments, but breaks in multi-instance environments — two server instances share no state, so a challenge issued by instance A can be replayed against instance B.

This PR adds RedisStore, a Redis-backed implementation of the Store protocol that solves this with atomic SET NX EX operations (no race conditions, no separate TTL call).

Usage:

from redis.asyncio import from_url
from mpp.stores.redis import RedisStore

store = RedisStore(await from_url("redis://localhost:6379"))

Includes: RedisStore + init.py, 7 tests, pympp[redis] optional dep.
Install with: pip install pympp[redis]