Submit per-user-headers values

Consumes a pending submission flow row: verifies the caller's values against the upstream MCP server, upserts the credential keyed by the flow row's (mode, identity), then deletes the flow row and the bound temp token. Mirrors the OAuth callback's "complete the flow" semantics.

Values for any key not in the live per_user_header_keys schema are dropped server-side so a stale UI can't persist deprecated keys.