fix(linkedin): avoid panic when the email response has no elements

[suvvvv]

What

linkedinProvider.GetUserData reads the user's email as e.Elements[0].HandleTilde.EmailAddress without checking that Elements is non-empty. When LinkedIn's /v2/emailAddress endpoint returns an empty array — e.g. when the email scope isn't granted or no primary email is available — this panics:

runtime error: index out of range [0] with length 0
  internal/api/provider/linkedin.go:128  (GetUserData)

The recoverer middleware turns the panic into a 500, so the OAuth callback fails instead of degrading gracefully. Note that getAvatarUrl a few lines above already guards its Elements access with len(...) > 0; the email path was missing the same check.

Fix

Read the email through a length-guarded local (mirroring getAvatarUrl). A missing email now yields an empty value that flows through the existing email != "" handling, so sign-in proceeds without the email rather than crashing.

Testing

Added TestSignupExternalLinkedin_MissingEmail in internal/api/external_linkedin_test.go, which drives the OAuth callback with an empty {"elements": []} email response. Before the change it fails with the recovered panic (500); after it, the callback redirects normally (302). All existing LinkedIn provider tests still pass; gofmt and go vet are clean.