⬆️ Bump firebase.google.com/go/v4 in /backend

Bumps [firebase.google.com/go/v4](https://github.com/firebase/firebase-admin-go) from 4.15.0 to 4.15.1.
- [Release notes](https://github.com/firebase/firebase-admin-go/releases)
- [Commits](firebase/firebase-admin-go@v4.15.0...v4.15.1)

---
updated-dependencies:
- dependency-name: firebase.google.com/go/v4
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

backend/go.mod

@@ -5,7 +5,7 @@ go 1.23
 require (
 	cloud.google.com/go/firestore v1.17.0
 	cloud.google.com/go/storage v1.47.0
-	firebase.google.com/go/v4 v4.15.0
+	firebase.google.com/go/v4 v4.15.1
 	github.com/99designs/gqlgen v0.17.57
 	github.com/form3tech-oss/jwt-go v3.2.5+incompatible
 	github.com/go-chi/chi/v5 v5.1.0

backend/go.sum

@@ -23,8 +23,8 @@ cloud.google.com/go/storage v1.47.0 h1:ajqgt30fnOMmLfWfu1PWcb+V9Dxz6n+9WKjdNg5R4
 cloud.google.com/go/storage v1.47.0/go.mod h1:Ks0vP374w0PW6jOUameJbapbQKXqkjGd/OJRp2fb9IQ=
 cloud.google.com/go/trace v1.11.2 h1:4ZmaBdL8Ng/ajrgKqY5jfvzqMXbrDcBsUGXOT9aqTtI=
 cloud.google.com/go/trace v1.11.2/go.mod h1:bn7OwXd4pd5rFuAnTrzBuoZ4ax2XQeG3qNgYmfCy0Io=
-firebase.google.com/go/v4 v4.15.0 h1:k27M+cHbyN1YpBI2Cf4NSjeHnnYRB9ldXwpqA5KikN0=
-firebase.google.com/go/v4 v4.15.0/go.mod h1:S/4MJqVZn1robtXkHhpRUbwOC4gdYtgsiMMJQ4x+xmQ=
+firebase.google.com/go/v4 v4.15.1 h1:tR2dzKw1MIfCfG2bhAyxa5KQ57zcE7iFKmeYClET6ZM=
+firebase.google.com/go/v4 v4.15.1/go.mod h1:eunxbsh4UXI2rA8po3sOiebvWYuW0DVxAdZFO0I6wdY=
 github.com/99designs/gqlgen v0.17.57 h1:Ak4p60BRq6QibxY0lEc0JnQhDurfhxA67sp02lMjmPc=
 github.com/99designs/gqlgen v0.17.57/go.mod h1:Jx61hzOSTcR4VJy/HFIgXiQ5rJ0Ypw8DxWLjbYDAUw0=
 github.com/BurntSushi/toml v0.3.1/go.mod h1:xHWCNGjB5oqiDr8zfno3MHue2Ht5sIBksp03qcyfWMU=