An MCP token drives every assistant tool; an API key reads trips over HTTP.
Handing the same credential to a third-party integration gives it the whole
MCP surface, so the two are now separate kinds on `mcp_tokens` and the kind
sits in the WHERE clause — a token of the wrong kind is indistinguishable
from one that does not exist.
Also adds `travellers` to the trip payload: display name and an owner flag,
no ids and no email addresses. An integration key reads its owner's
itinerary; it is not a way to enumerate the people around them.
Documents the surface in the wiki, since integrators need something to read
that is not the source.
Maurice