Redis production hardening

[an8kk]

Adds a production-focused hardening pass after v0.1.0-alpha.

Changes:

• documents Redis persistence, eviction, replication, cluster, and Lua caveats
• adds cluster-safe Redis key generation with shared hash tags
• switches Redis Lua transitions to cached script objects
• adds task attempt tracking
• adds queue DeadLetter support
• adds memory and Redis DLQ baseline
• adds EngineConfig with MaxAttempts and RequeueRetryDelay
• keeps default infinite retry behavior
• adds ACK/expiration race coverage
• adds Redis lists vs Streams ADR
• updates MOXY.STATS over RESP to include dead count

Verification:

• go mod tidy
• go vet ./...
• go test ./...
• go test ./internal/queue -count=100
• go test ./internal/core -count=100
• go test ./internal/service -count=100
• go test ./internal/command -count=100
• go test ./internal/protocol -count=100
• go test ./internal/server -count=100
• go test ./internal/resp -count=100

Manual smoke:

• PING -> PONG
• MOXY.ENQUEUE jobs hello -> task_id
• MOXY.FETCH jobs 30000 -> lease_id/task_id/payload
• MOXY.STATS jobs -> ready 0 processing 1 dead 0 active_leases 1 heap 1

Limitations:

• Redis integration tests were not run successfully in this environment because Redis was unavailable on localhost:6379.
• No Streams backend.
• No WAL.
• No snapshots.
• No full Redis proxying.
• No production-readiness claim.