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/architecture-class-diagram.puml

@@ -188,11 +188,6 @@ package "Models" #D3D3D3 {
 }
 
 package "Utils" #DDA0DD {
-    class JWT {
-        ' +signToken()
-        ' +verifyToken()
-    }
-
     class Password {
         ' +hash()
         ' +compare()
@@ -215,8 +210,6 @@ ChatController --> CaldavService
 EventController --> EventService
 EventController --> CaldavService
 CaldavController --> CaldavService
-AuthMiddleware --> JWT
-
 ' Service -> Interfaces (intern)
 AuthService --> UserRepository
 ChatService --> ChatRepository
@@ -227,7 +220,6 @@ CaldavService --> CaldavRepository
 CaldavService --> EventService
 
 ' Auth uses Utils
-AuthService --> JWT
 AuthService --> Password
 
 ' Event/Chat uses Utils