The identity forwarded to opt-in MCP servers was a one-dimensional string
(ChatOrigin.requesterId): a MateClaw username for web logins, but a webchat
visitorId for visitors and an IM sender id for IM — indistinguishable to the
REST backend. The signed-token mode (d204b702) made this worse: an RS256
signature over an unauthenticated visitorId reads as "MateClaw authenticated
this user" to any backend that trusts the signature.
Introduce an identity-typing dimension at McpIdentityForwardService:
- classify() branches on ChatOrigin: authenticated (web login, sub=immutable
userId), anonymous (webchat visitor, trust=anonymous), external (IM sender,
trust=external), or none (cron/system → nothing injected, fail-closed).
- mint() adds `trust` and `channel_type` claims; plaintext value is prefixed
`trust:subject` so backends can tell the kinds apart without a JWT.
The immutable userId reaches resolve() without coupling it to the user store:
JwtAuthFilter stamps user.id into auth.setDetails() (both JWT and PAT paths),
and ChatController.memoryOrigin carries it on a new ChatOrigin.requesterUserId
field (only-add, per the record's evolution rule).
Resolves the webchat semantic mismatch raised in #459 and the "sub should be
an immutable user id" follow-up. 82 tests green (4 identity classes covered
with claim assertions + full ChatOrigin/MCP regression).
(cherry picked from commit b5d2cfbf98b39848d7139c743a0b81fea71e8ffe)
