fix(c4): sequence diagram — username → email in auth flow

Three stale references: "Enter username + password", Base64 encode
"user:password", and SELECT WHERE username — all updated to email to
match AppUserRepository.findByEmail() and CustomUserDetailsService.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/architecture/c4-diagrams.md

@@ -517,12 +517,12 @@ sequenceDiagram
     participant Backend as Backend (Spring Boot)
     participant DB as PostgreSQL
 
-    User->>Browser: Enter username + password
+    User->>Browser: Enter email + password
     Browser->>Frontend: POST /login (form action)
-    Frontend->>Frontend: Base64 encode "user:password"
+    Frontend->>Frontend: Base64 encode "email:password"
     Frontend->>Backend: GET /api/users/me<br/>Authorization: Basic <token>
     Backend->>Backend: Spring Security parses Basic Auth
-    Backend->>DB: SELECT user WHERE username=?
+    Backend->>DB: SELECT user WHERE email=?
     DB-->>Backend: AppUser + groups + permissions
     Backend->>Backend: BCrypt.matches(password, hash)
     Backend-->>Frontend: 200 OK — UserDTO