Add support for sorted set ZSET operations

[oliverhaas]

Problem Statement

Django-redis supports set operations but not sorted sets, forcing you to use the raw Redis client and lose serialization/connection management.

Describe the solution you'd like

Add sorted set operations (zadd, zcard, zcount, zincrby, zpopmin/zpopmax, zrange/zrangebyscore, zrank, zrem, zremrangebyscore, zrevrange, zrevrangebyscore, zscore) with proper serialization, zadd flags support, and key versioning.

Describe alternatives you've considered

Raw redis-py loses serialization. Custom backend or external package is overkill.

Additional context

Have a working mixin implementation. 336 tests, all checks passing. Ready to PR in a second as a PoC. Feedback appreciated.