Merge a contact

Collapses two auto-created contacts for the same person into one. The survivor is the {contactId} in the path; the source_contact_id in the body is absorbed into it. Every identity on the source is re-parented onto the survivor, tags are unioned, the survivor keeps its own name (falling back to the source's), and the source becomes a deprecated tombstone stamped with merged_into. Because chats and messages reference identities (not contacts), all conversations and timeline entries follow the moved identities automatically — nothing is rewritten or lost, and reads such as the survivor's timeline immediately reflect the merge. Idempotent: re-merging the same source into the same survivor returns 200; a source already merged into a different contact returns 409. Emits a contact.merged webhook event.