optional redis storage backend so multiple instances can share state

[Mzack9999]

For #1267. Lets you run multiple interactsh-server instances behind a load balancer sharing one Redis. Pass --redis-url (and optionally --redis-prefix) to enable; everything else stays exactly as it is today, the existing in-memory/LevelDB path is left alone.

The new backend plugs into the same storage.Storage interface, uses hash-tagged keys so it also works on Redis Cluster, and runs the per-consumer read/evict/trim ops as atomic Lua scripts. TTL respects the existing sliding/fixed eviction strategies.

Tested with miniredis-based unit tests, an opt-in integration suite against a real Redis, and a two-instance smoke harness under deploy/redis-test/ that registers via pkg/client on instance A, fires an HTTP hit at B, and confirms it polls back from A.

ACME certs and per-instance metrics still live on each box, that's a separate problem.

Closes #1267

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