Complete GORM to sqlc migration

- Remove GORM dependency, use sqlc for type-safe SQL queries
- Update all handlers to use sqlc patterns (context, value types)
- Fix N+1 query problem in domain listing with JOIN query
- Enable SQL query logging in debug mode (USE_EMBEDDED=false)
- Add comprehensive comments for non-Go developers

backend/internal/api/handlers/auth.go

@@ -49,8 +49,6 @@ func (h *AuthHandler) Login(c *gin.Context) {
 		return
 	}
 
-	ip := h.getClientIP(c)
-
 	user, err := h.db.GetImcUserByUsername(c.Request.Context(), req.Username)
 	if err != nil || !auth.CheckPassword(req.Password, user.PasswordHash) {
 		Error(c, http.StatusUnauthorized, "invalid credentials")