fix(models): guard identity email against non-string metadata

[suvvvv]

What

NewIdentity in internal/models/identity.go asserted identityData["email"] to a string without checking the type:

if email, ok := identityData["email"]; ok {
    identity.Email = storage.NullString(email.(string)) // panics if not a string
}

identityData is assembled by merging user-supplied signup data metadata into provider.Claims. For a phone signup the email claim is empty and Claims uses structs:"email,omitempty", so it is dropped from the map — which lets a user-supplied data.email that is null (or any non-string) survive into identityData["email"]. The unchecked assertion then panics:

panic: interface conversion: interface {} is nil, not string
  internal/models/identity.go:63  (NewIdentity)
  internal/api/external.go:907    (createNewIdentity)
  internal/api/signup.go:218
  internal/api/otp.go:181 / :91   (SmsOtp → Signup)

The panic happens during identity creation, before any SMS is sent, so the request returns a bare 500 unexpected_failure instead of a meaningful error. This matches the stack trace and the "happens on phone signup / SMS OTP" symptom in the issue.

Fix

Only assign the identity email when the value is actually a string, and apply the same guard to the assertion in BeforeUpdate. A non-string email is ignored (the identities.email column is optional), so signup proceeds normally instead of crashing.

Testing

Added regression coverage in internal/models/identity_test.go for a string email and for non-string values (nil, int, bool, map) that previously panicked. go test ./internal/models/... passes; gofmt and go vet are clean.

Fixes #2268