feat: implement gRPC server + grpc-gateway wiring (task 9)

Assembles tasks 1/6/7/8 into a runnable cmd/server binary: QueueServiceServer
handlers translating kafkamgmt.v1 proto to internal/core/queue's plain Go
types, a lazy per-queue Kafka consumer registry for ReceiveMessage, and a
Redis-scan-based queue discovery loop that starts a reaper goroutine per
queue (queue lifecycle isn't exposed over gRPC, so this is the server's only
signal). Promotes kmsvc-proto to a direct go.mod dependency.

Handler-level integration tests run against kfake+miniredis (same documented
tradeoff as tasks 5-7's envtest/testcontainers substitution), exercising
send->receive->delete through the real QueueServiceServer implementation.

internal/redis/queuemeta.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/redis/go-redis/v9"
@@ -79,3 +80,19 @@ func DeleteQueueMeta(ctx context.Context, rdb *redis.Client, queue string) error
 	}
 	return nil
 }
+
+// ListQueueNames scans for every queue with a published `kmsvc:queue:` row,
+// used by the message-plane server to discover which queues need a reaper
+// goroutine (design.md §5) since queue lifecycle isn't exposed over gRPC.
+func ListQueueNames(ctx context.Context, rdb *redis.Client) ([]string, error) {
+	const prefix = "kmsvc:queue:"
+	var names []string
+	iter := rdb.Scan(ctx, 0, prefix+"*", 0).Iterator()
+	for iter.Next(ctx) {
+		names = append(names, strings.TrimPrefix(iter.Val(), prefix))
+	}
+	if err := iter.Err(); err != nil {
+		return nil, fmt.Errorf("list queue names: %w", err)
+	}
+	return names, nil
+}