Add linkedin link to profile: BE

[joelamouche]

Similar to #73

• Add linkedin_account to profile table in db
• Enforce global, case-insensitive uniqueness. First-come-first-serve: once claimed, cannot be claimed by another profile unless changed by admin tooling later.

API

Reuse existing profile endpoints.
PUT /profile (existing): accept optional linkedin_account field in the payload.
Normalize input for checks: trim and to_lowercase for uniqueness evaluation only.
Persist original input (with original casing).
On conflict with another profile’s handle, return 409.
On invalid format, return 400.
GET profile endpoints (existing): include linkedin_account in returned profile DTOs (subject to current visibility rules).