refactor: remove all JWT-related code and references

JWT was never used - auth uses X-User-Id header. Removes jwt.ts utility,
jsonwebtoken dependency, stubbed refresh/logout endpoints, and updates
all docs (PUML diagrams, api-routes, tex, CLAUDE.md) accordingly.

docs/component-diagram.puml

@@ -77,7 +77,6 @@ package "apps/server (Express.js)" as ServerPkg #98FB98 {
     }
 
     package "Utils" {
-        [JWT] as JWTUtil
         [Password] as PwdUtil
         [RecurrenceExpander] as RecExpander
         [EventFormatters] as EvtFormatters
@@ -154,9 +153,7 @@ GPT ..|> Interfaces
 Repos ..|> Interfaces
 
 ' Backend: Service -> Utils
-AuthSvc --> JWTUtil
 AuthSvc --> PwdUtil
-Middleware --> JWTUtil
 EventSvc --> RecExpander
 ChatSvc --> EvtFormatters