fix: repair Mermaid diagram syntax errors and add PRD task plan

01 - Architecture/Security Architecture.md

@@ -47,11 +47,13 @@ sequenceDiagram
     DB-->>BE: user doc (incl. hashed password)
     BE->>BE: bcrypt.compare(password, hash)
     alt invalid
-        BE->>DB: increment loginAttempts; lock at N
+        BE->>DB: increment loginAttempts
+        BE->>DB: lock account at N
         BE-->>FE: 401 / 423 locked
     else valid
         BE->>BE: sign JWT (7d), refresh (30d)
-        BE->>DB: store refresh-token id; clear attempts
+        BE->>DB: store refresh-token id
+        BE->>DB: clear attempts
         BE-->>FE: 200 { user, token, refreshToken }
     end
 ```