fix: playout Dockerfile use existing go.mod instead of init

backend/playout/Dockerfile

@@ -2,11 +2,10 @@ FROM golang:1.25-alpine AS builder
 
 WORKDIR /app
 COPY src/ ./src/
+COPY src/go.mod src/go.sum ./
 
-# Initialize Go module if not present
-RUN go mod init playout || true
-RUN go mod tidy
-
+# Download dependencies and build
+RUN go mod download
 RUN CGO_ENABLED=0 GOOS=linux go build -o /playlist-manager ./src/playlist.go
 
 FROM alpine:3.20