refactor: extract duplicated annotation filepath helper

[aftersnow]

Description

The logic to extract filepath from layer annotations is duplicated in 3+ files:

• pkg/backend/fetch.go
• pkg/backend/fetch_by_d7y.go
• pkg/backend/pull_by_d7y.go

var annoFilepath string
if desc.Annotations != nil {
    if desc.Annotations[modelspec.AnnotationFilepath] != "" {
        annoFilepath = desc.Annotations[modelspec.AnnotationFilepath]
    } else {
        annoFilepath = desc.Annotations[legacymodelspec.AnnotationFilepath]
    }
}

Suggested Fix

Extract into a shared helper in the backend package:

func getAnnotationFilepath(annotations map[string]string) string {
    if annotations == nil {
        return ""
    }
    if path := annotations[modelspec.AnnotationFilepath]; path != "" {
        return path
    }
    return annotations[legacymodelspec.AnnotationFilepath]
}

Context

Found during code review of #468. This is a pre-existing pattern, not introduced by that PR.