Add Redis integration

[barton87]

Summary

Adds a Redis integration for Plakar, covering the bounty requirements from PlakarKorp/hub#3:

• connect to local/remote Redis using redis-cli
• trigger BGSAVE and wait for completion by default
• ingest the resulting RDB as /dump.rdb in the Plakar snapshot
• restore /dump.rdb back to a filesystem path Redis can load on startup

The importer supports redis:// and rediss:// connection URIs, ACL username/password, TLS flags, custom redis-cli location, configurable BGSAVE behavior, and a remote fallback via redis-cli --rdb when the local RDB path is not readable.

The exporter uses redis-file:// / output= to write the RDB file and refuses overwrite unless force=true.

Verification

Run locally from redis/:

• go mod tidy
• go test ./...
• make build

All passed.

Fixes PlakarKorp/hub#3.