Remove self-service legacy membership claim flow

[markgoho]

Summary

Remove the obsolete self-service legacy membership claim flow. Legacy membership migration is now admin-only via Link Legacy Membership.

Context

See docs/adr/0002-legacy-membership-migration-admin-only.md.

The repo still exposes a self-service claim route in the profiles API, but current product/domain language rejects member self-claim and automatic linking. The cooperative has roughly 65 members and legacy migrations happen slowly enough that admin linking is preferred.

Scope

• Remove POST /api/profiles/:slug/claim from the active profiles API surface.
• Remove claim route logic, tests, service interfaces, and dependencies if no longer used.
• Check whether the members app still calls the claim endpoint; remove or replace that UI path if present.
• Keep admin-only legacy linking workflows intact.
• Update docs/tests to use Link Legacy Membership language instead of claim language.

Acceptance criteria

• No member-facing or unauthenticated path can claim a legacy membership.
• Admin legacy membership linking still works.
• Tests cover the remaining admin-only behavior at user-facing boundaries.
• No references imply self-service claim is current product behavior.