fix(oci): check for existing manifest before insert to fix re-push FK violation

[The127]

Summary

• After the upsert fix in fix(oci): upsert manifests and tags on push to handle re-pushes #53, manifest re-pushes still panicked with interface conversion: interface {} is uint32, not uint followed by a FK violation
• Root cause: the tag was queued with manifest.GetId() (new UUID) before SaveChanges ran; the manifest upsert returned the existing row's id via SetId, leaving the tag referencing a UUID that was never inserted
• Fix: query for the existing manifest first — if found, reuse it and skip the insert so the tag always references a valid manifest id
• Plain INSERT is restored for manifests (check handles sequential re-pushes; DB unique constraint handles concurrent races)
• Tag upsert from fix(oci): upsert manifests and tags on push to handle re-pushes #53 is kept so re-pushing a tag updates it to the latest manifest

Test plan

• Push same image twice — second push succeeds
• Push different image to same tag — tag resolves to new manifest on pull