feat(redis): add shard-aware key schema, Lua scripts, and atomic ops

Implements the kmsvc: key schema (inflight, pending/watermark keyed by
shard+partition, vis_index, dedup, fifo_lock, queue meta, shard map) plus
the atomic reap/ack Lua scripts used for safe multi-replica redelivery
and DLQ routing.

internal/redis/keys.go

@@ -0,0 +1,53 @@
+// Package redis implements the key schemas and atomic Lua scripts from
+// design.md §4/§5, shared by the queue-operator and the message-plane service.
+package redis
+
+import "fmt"
+
+func InFlightKey(queue, receiptHandle string) string {
+	return fmt.Sprintf("kmsvc:inflight:%s:%s", queue, receiptHandle)
+}
+
+func PendingKey(queue, shardID string, partition int32) string {
+	return fmt.Sprintf("kmsvc:pending:%s:%s:%d", queue, shardID, partition)
+}
+
+func WatermarkKey(queue, shardID string, partition int32) string {
+	return fmt.Sprintf("kmsvc:watermark:%s:%s:%d", queue, shardID, partition)
+}
+
+func VisIndexKey(queue string) string {
+	return fmt.Sprintf("kmsvc:vis_index:%s", queue)
+}
+
+func DedupKey(queue, groupID, dedupID string) string {
+	return fmt.Sprintf("kmsvc:dedup:%s:%s:%s", queue, groupID, dedupID)
+}
+
+func FIFOLockKey(queue, groupID string) string {
+	return fmt.Sprintf("kmsvc:fifo_lock:%s:%s", queue, groupID)
+}
+
+func QueueMetaKey(queue string) string {
+	return fmt.Sprintf("kmsvc:queue:%s", queue)
+}
+
+func ShardMapKey(queue string) string {
+	return fmt.Sprintf("kmsvc:shardmap:%s", queue)
+}
+
+func ShardMapChannel(queue string) string {
+	return fmt.Sprintf("kmsvc:shardmap_invalidate:%s", queue)
+}
+
+func QueueMetaChannel(queue string) string {
+	return fmt.Sprintf("kmsvc:queuemeta_invalidate:%s", queue)
+}
+
+func RedeliverKey(queue string) string {
+	return fmt.Sprintf("kmsvc:redeliver:%s", queue)
+}
+
+func QueueLockKey(queue string) string {
+	return fmt.Sprintf("kmsvc:queue_lock:%s", queue)
+}